Indoor grilled summer veggie sandwich - delicious summer grilled veggies (grilled indoor) made into a sandwich that everyone can enjoy on Labor day weekend!

Summer always feels like it starts with Memorial Day and wraps up with Labor Day. And in between, there’s plenty of good food on the grill. Since moving to LA, backyard grilling isn’t really an option for me, so I turned to my trusty cast iron grill pan. It gives me all those delicious charred veggies right at home (and yes, in the comfort of air conditioning 😉)

Why You’ll Love This Recipe

All the grilled flavor, indoors – no backyard grill required, just a cast iron pan.
Simple balsamic marinade – adds tangy, herby flavor that works with any veggie.
Flexible and versatile – pile into a sandwich, toss into a grain bowl, or serve as a side.
Great for meal prep – make a big batch and use the leftovers throughout the week.
Vegetarian-friendly crowd pleaser – everyone loves smoky, charred veggies.

zucchini and yellow squash with balsamic vinegar marinade

How to Make Indoor Grilled Veggie Sandwich

Mix the Marinade - Whisk balsamic vinegar, olive oil, garlic, Dijon mustard, and Italian seasoning in a small bowl.
Prep the Veggies - Slice zucchini, yellow squash, eggplant, mushrooms, onion, and peppers. Toss with a few spoonfuls of the marinade.
Heat the Pan - Warm a cast iron grill pan over medium-high heat and brush lightly with neutral oil.
Grill in Batches - Cook vegetables in a single layer, 2–5 minutes per side depending on the veggie, until tender with grill marks. Sprinkle with salt as you go.
Assemble the Sandwich - Cut the focaccia in half, spread with pesto, layer in the grilled veggies, and slice into portions.

Helpful Tips for the Best Indoor Grilled Veggies

Preheat your pan well: Give your cast iron grill pan at least 4–5 minutes to heat up. A hot pan is key to those beautiful grill marks.
Brush the pan, not the veggies: Lightly brush the grill pan with oil instead of drenching the vegetables. This prevents sogginess and keeps the char crisp.
Cut evenly: Slice vegetables about ¼ inch thick so they cook at the same rate.
Flip once: Place veggies down and let them be — flipping just once helps build those bold grill marks.
Season at the end: Sprinkle with salt after grilling so the vegetables stay juicy and don’t release too much water.
Batch cook if needed: Don’t overcrowd the pan; cook in batches for the best results.

Ways to Enjoy These Grilled Vegetables

These balsamic grilled vegetables are incredibly versatile, and you can use them in all kinds of Mediterranean-inspired meals. If you’d like a quicker option, check out my Perfect Air Fryer Veggies recipe for a weeknight shortcut.

For sandwiches, this focaccia version is hearty and satisfying, but if you’re craving another flavor profile, try my Eggplant Halloumi Sandwich. The grilled veggies also make a delicious topping for my Mediterranean Flatbread.

For a full dinner, serve them as a side with my Couscous Risotto with Shrimp and Leek or pair them with Greek Chicken Meatballs. And if you’d like to keep things vegetarian, tuck them into a wrap, add them to a grain bowl, or serve them alongside my Zucchini Couscous.

This indoor grilled veggie sandwich is my Mediterranean-inspired take on a classic. It’s layered with smoky, balsamic-marinated vegetables and plenty of pesto on soft focaccia. Hearty enough for dinner, light enough for lunch, and endlessly versatile with leftovers. If you enjoy flavors like this, check out my Mediterranean Pantry Essentials Guide to stock up on the ingredients that make recipes like this simple and delicious.

Labor Day Vegan Veggie Sandwich close up

Indoor Grilled Summer Veggie Sandwich

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Indoor grilled summer veggie sandwich - delicious summer grilled veggies (grilled indoor) made into a sandwich that everyone can enjoy on Labor day weekend!

Author: Georgie
Prep Time: 15 min
Cook Time: 60 minutes
Total Time: 1 hour 15 minutes
Yield: 12 mini Sandwichs 1x
Category: Main course
Method: Stovetop
Cuisine: American
Diet: Vegetarian

Ingredients

Scale

For Balsamic Marinade

¼ cup balsamic vinegar
¼ cup extra virgin olive oil
4 cloves garlic, minced
2 tsp Dijon mustard
2 tsp Italian seasoning
Salt, to taste

For Vegetables

1 large zucchini, sliced diagonally ¼-inch thick
1 large yellow squash, sliced diagonally ¼-inch thick
2 small Japanese eggplants, sliced diagonally ¼-inch thick
3 large king oyster mushrooms, sliced lengthwise
1 medium red onion, sliced into rounds (secure with toothpicks if needed)
6 mini sweet peppers, seeded and halved lengthwise
2–3 tablespoon neutral oil (avocado or canola), for brushing grill pan and vegetables

For Sandwich

1 focaccia bread, cut horizontally
2–3 tablespoon pesto of your choice (I used Trader Joe’s Vegan Kale, Cashew & Basil Pesto)
Grilled vegetables from above

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Instructions

Make Marinade
1. In a small bowl, whisk together balsamic vinegar, olive oil, garlic, Dijon, Italian seasoning, and a pinch of salt.
Grill the Vegetables
1. Heat a cast-iron grill pan over medium-high heat for 4–5 minutes until hot. Brush lightly with neutral oil.
2. Toss each type of vegetable in a few spoonfuls of the balsamic marinade. Brush lightly with oil if pieces look dry.
3. Grill vegetables in a single layer, without moving them, until tender and grill marks appear:
  - Zucchini & yellow squash: 3–4 minutes per side
  - Eggplant: 3–4 minutes per side
  - King oyster mushrooms: 3–4 minutes per side
  - Mini sweet peppers: 2–3 minutes per side
  - Red onion rounds: 4–5 minutes per side
4. Transfer grilled veggies to a platter and sprinkle with extra salt if needed.
Assemble Sandwich
1. Cut focaccia horizontally. Spread pesto on both cut sides of the bread.
2. Layer grilled vegetables generously on the bottom half.
3. Top with the other half of the focaccia. Slice into sandwich portions or sliders, securing with toothpicks if needed.

Notes

Outdoor Grill Option: After flipping the vegetables the first time, brush them with more marinade, sprinkle with salt, and cook 1–2 minutes longer. Cook time will vary depending on your grill temperature and how soft or crisp you like your veggies — I prefer them slightly crispy.
Cooking Time: This recipe takes longer than many of my others because I use a medium cast-iron grill pan and cook in multiple batches. If you have a larger pan, you can shorten the time.
Serving & Leftovers: You’ll likely have extra grilled vegetables — they’re delicious as a side dish for any grilled meal or for making additional sandwiches.