2 tablespoons extra-virgin olive oil

1 pound sweet Italian sausages, casings removed

1 small yellow onion, finely chopped

4 garlic cloves, minced

Red-pepper flakes

1 teaspoon dried oregano

Two 28-ounce cans whole peeled tomatoes

1 basil sprig, plus a handful of basil leaves

Salt and freshly ground black pepper

One 9-ounce package no-boil lasagna noodles (such as Barilla brand)

4 ounces mascarpone cheese or cream cheese

½ pound fresh mozzarella, thinly sliced and patted dry