Thinly slice the onion and mushrooms. Shred or thinly slice the cooked chicken if not already prepared. Grate the Parmesan and shred or slice your choice of melting cheese. Chop parsley if using.

Cook Filling

Heat a large skillet over medium-high heat. Add 1 tablespoon olive oil and 1 tablespoon butter. When butter melts and oil shimmers, add the sliced onions. Sauté 4–5 minutes, stirring occasionally, until they begin to soften and turn translucent.

Add the sliced mushrooms to the pan with a pinch of salt. Cook 4–5 minutes more, stirring, until mushrooms release their moisture and begin to brown.

Stir in the minced garlic, dried oregano, and dried thyme. Cook 30–45 seconds until fragrant.

Add the shredded chicken to the skillet. Season with kosher salt, freshly ground black pepper, and red pepper flakes if using. Toss to combine and cook 2–3 minutes until chicken is heated through.

If the pan looks dry or bits are sticking, add 1–2 tablespoons low-sodium chicken broth to deglaze the pan, scraping up browned bits. Cook 1 minute more to reduce any excess liquid. Stir in the grated Parmesan so it melts slightly and coats the filling. Remove pan from heat.

Assemble & Toast Sandwiches

Preheat oven broiler to high (or set a countertop broiler/panini press). Split the hoagie rolls horizontally, keeping one side attached if desired, and lightly butter the cut sides with the remaining 1 tablespoon butter.

Place the bottoms of the rolls on a baking sheet. Evenly mound the chicken, mushroom and onion mixture onto each roll bottom, dividing filling between the two rolls.

Sprinkle the shredded/sliced choice of melting cheese evenly over the hot filling so it begins to melt. If you want extra Parmesan, sprinkle a little more on top.

Place the assembled sandwiches under the broiler about 4–6 inches from heat for 2–4 minutes, watching closely, until the cheese is melted and the bread edges are toasted. Alternatively, toast in a 400°F (200°C) oven for 6–8 minutes or press in a hot panini press until cheese melts and bread is golden.

Remove from oven, sprinkle with chopped parsley if using, and place the top halves of the rolls onto each sandwich. Serve hot.