Place halved/torn mushrooms in a large bowl. Drizzle with 2 tablespoons olive oil, 1 tablespoon soy sauce, 1 teaspoon balsamic vinegar, and add minced garlic and thyme. Toss gently to coat evenly.

Spread mushrooms in a single layer on the prepared baking sheet. Sprinkle with 3/4 teaspoon salt and 1/2 teaspoon freshly ground black pepper (reserve remaining salt to taste). Avoid overcrowding — use two sheets if necessary.

Roast in the preheated oven for 15–20 minutes, stirring once halfway through, until mushrooms are deeply browned at the edges and have released their juices. If mushrooms release a lot of liquid, remove from oven after 12–15 minutes and increase heat to 450°F (230°C) for 3–5 minutes to encourage browning.

Remove from oven and taste; season with remaining salt if needed. Let rest 5 minutes — mushrooms will continue to concentrate flavor.

Optional Caramelized Onions

While mushrooms roast (or up to a day ahead), heat 1 tablespoon butter or olive oil in a medium skillet over medium-low heat. Add sliced onions and 1/4 teaspoon salt and cook, stirring occasionally, for 20–25 minutes until soft and golden. If you want sweeter, stir in 1/2 teaspoon brown sugar about halfway through cooking. Reduce heat if onions begin to brown too quickly.

When onions are deeply golden and sweet, remove from heat and set aside. Keep warm or refrigerate up to 3 days.

Sub Assembly and Finish

Lightly toast sub rolls: spread softened butter on cut sides and toast in a skillet over medium heat or under a broiler until golden, about 1–2 minutes. Watch carefully to avoid burning.

Mix mayonnaise and Dijon mustard in a small bowl. Spread about 1 tablespoon of the mayo-mustard mixture on each bottom half of the toasted rolls and a thin layer on the top halves if desired.

Place a slice of provolone (or vegan cheese) on each bottom half. Divide the roasted mushrooms evenly among the four subs, piling them on top of the cheese so the residual heat softens/melts it slightly.

Top the mushrooms with about 1/4 cup caramelized onions per sandwich, 1/4 cup roasted red peppers, a handful (about 1/2 cup) of arugula or lettuce, and a few pickled banana peppers or pickles to taste.

Close the sandwiches, press gently, and if you like, return to a warm skillet or sandwich press for 1–2 minutes to meld flavors and finish melting the cheese.

Slice each sub in half and serve immediately. Serve extra pickles or hot sauce on the side.