Instructions

• Put the all-purpose flour, the beaten egg, and the panko breadcrumbs in three separate shallow bowls for dredging.
• Pat the 2 thin cutlets dry. Sprinkle about half of the kosher salt (from the 1/4 to 1/2 teaspoon) and about half of the 1/4 teaspoon freshly ground black pepper evenly over both cutlets.
• Dredge each cutlet in flour (shake off excess), then dip in the beaten egg, then coat with panko breadcrumbs (shake off excess). Do not press the breadcrumbs into the meat and do not refrigerate the breaded cutlets.
• Heat a large nonstick skillet over medium–high heat until hot. Add the breaded cutlets to the dry skillet and cook until golden and cooked through, about 1–3 minutes per side for 1/4-inch cutlets (watch closely). Transfer cooked cutlets to a plate and keep warm while you cook the vegetables; do not tent with foil.
• If there are large browned bits in the pan, wipe them out with a paper towel; leave any thin film of fond for flavor.
• Return the skillet to medium heat. Add the halved, thinly sliced yellow onion and cook, stirring occasionally, until lightly browned, about 6–8 minutes.
• Add the thinly sliced red, green, and yellow bell peppers and the sliced mushrooms to the skillet. Sprinkle the vegetables with the garlic powder (1/4 teaspoon), onion powder (1/4 teaspoon), the remaining kosher salt, and the remaining black pepper. Cook, stirring occasionally, until the peppers are crisp‑tender, about 4–5 minutes.
• Divide the cooked vegetable mixture into four equal portions in the skillet or transfer to a plate and portion into four piles in the skillet. Top each portion with one slice of Provolone or Swiss (one slice per portion).
• Cover the skillet briefly (use a lid) until the cheese melts, about 1–2 minutes.
• Spread mayonnaise and grainy mustard on both sides of each lightly toasted hoagie roll.
• Cut each cooked cutlet into two equal pieces so you have four meat portions. Place one cutlet portion on the bottom half of each roll, top with a cheese-covered portion of the vegetables, then close the roll.
• Serve the sandwiches immediately.