This Italian version of the famous Philly cheesesteak sandwich is overflowing with thinly sliced skirt steak, garlicky onions and peppers, nutty fontina cheese, and homemade basil pesto.
Heat 2tbsp of the oil in a frying pan over medium heat. Add the bell pepper, onion, and garlic and cook, stirring, until tender, 5–8 minutes. Remove from the heat, stir in the vinegar, and season with salt and pepper. Set aside.