Preheat the oven to 425°F (220°C). Clean mushrooms by wiping with a damp cloth or quickly rinsing and patting dry. Trim stems if needed and halve or quarter larger mushrooms so pieces are roughly uniform (bite-sized).

Season

Place prepared mushrooms in a large bowl. Add 2 tbs olive oil, 1 tbs melted butter, minced garlic, fresh thyme leaves, soy sauce, balsamic vinegar, 1 tsp salt, and 1 tsp freshly ground black pepper. Toss thoroughly so mushrooms are evenly coated.

Roast

Spread mushrooms in a single layer on a rimmed baking sheet. Roast in the preheated oven for 18–22 minutes, stirring once halfway through, until mushrooms are deeply browned at the edges and most of the liquid has evaporated. If pieces are small, check at 15 minutes to avoid overcooking.

Finish

Remove mushrooms from oven and taste; adjust seasoning with a pinch of salt or pepper if needed. Let mushrooms rest for 3–5 minutes — any remaining pan juices will concentrate and coat the mushrooms, enhancing flavor.

Sub Assembly

Prep rolls and condiments

While mushrooms roast, split the sub rolls lengthwise without cutting all the way through, or fully split if you prefer. Lightly brush the cut sides with 1 tsp olive oil. Place rolls cut-side down on a separate baking sheet and toast in the oven for 3–5 minutes until slightly golden and crisp. In a small bowl, combine mayonnaise and Dijon mustard; spread this mixture evenly on both cut sides of the warm rolls.

Layer cheese and mushrooms

Place one or two slices of provolone or cheddar on the bottom half of each toasted roll. Pile a generous portion (about half of the roasted mushrooms) on top of the cheese so residual heat melts it slightly. If you want extra melted cheese, return assembled bottoms to the oven for 1–2 minutes until cheese softens.

Add vegetables and finish

Top the mushrooms with roasted red peppers, thinly sliced red onion, and a handful of baby arugula or mixed greens. Close the sandwich and press gently. Cut each sub in half if desired and serve immediately while warm.

Serving suggestion

Serve the sandwiches with pickles, potato chips, or a simple side salad. Leftover mushrooms keep refrigerated up to 3 days and make excellent reheated sandwich filling.