Baked Polenta with Sausage and Tomato-Pepper Sauce

olive oil

crushed tomatoes

chopped fresh oregano

medium yellow onion, halved lengthwise, then cut into thin wedges

medium yellow, red, or orange bell pepper, cut lengthwise into thin slices

garlic, minced

each salt, freshly ground black pepper, and red chile flakes

bulk Italian sausage, sweet or spicy (or 1 lb. Italian sausage links removed from their casings)

tubes (16 oz. each) prepared polenta, sliced into 1/2-in.-thick rounds

fresh mozzarella, drained and sliced into 1/4-in.-thick rounds