An easy cheese filling

Perfect for summer sandwiches

Ingredients

• 2 kilos boneless beef steak or flank steak, very thinly sliced 
• 1½ teaspoons salt
• ½ teaspoon freshly ground pepper
• ½ kilo mushrooms, quartered
• 3 tablespoons extra-virgin olive oil
• 1 medium yellow onion, thinly sliced
• 1 green bell pepper, thinly sliced
• 1 red bell pepper, thinly sliced
• 4 garlic cloves, minced, or fewer to taste
• 2 cups shredded mozzarella
• ½ cup grated Parmesan
• 4 scallions, thinly sliced (optional)
• 6 long rolls, toasted (optional)

Step-by-step instructions

1. Season the steak all over with 1 teaspoon salt and pepper.
2. Heat a medium oven-safe skillet over medium-high heat and add the mushrooms to the dry skillet. Sprinkle with the remaining ½ teaspoon salt and cook until browned, about 5 minutes. 
3. Increase the heat to high and add 2 tablespoons olive oil to the skillet. When the oil shimmers, add the onions and peppers and cook, stirring occasionally, until they begin to soften, 3-5 minutes. Transfer the vegetables to a plate.
4. In the same skillet, add the remaining 1 tablespoon olive oil over high heat. Once the oil shimmers, add the steak in small batches and cook for 1-2 minutes per side, until browned. Transfer to a plate.
5. Repeat with the remaining steak. 
6. Reduce the heat to medium, add the garlic, and cook, stirring, until fragrant, about 1 minute more.
7. Return the vegetables and steak, along with any accumulated juices, to the skillet and stir to combine.
8. Cover evenly with mozzarella and Parmesan. 
9. Turn on the broiler with a rack in the middle position. Place the skillet in the oven and broil until the cheese melts and bubbles. Sprinkle with scallions, if desired.
10. Serve the filling hot, divided between the rolls, or serve it with French fries.