Preheat the oven to 375°F (190°C).
Bring a large pot of salted water to a boil and cook the ravioli according to package directions, boiling for one minute less than the package recommends. Drain and set aside.
Arrange four oven-safe ramekins (about 5 inches across and 3 inches deep) on a baking sheet.
Spoon a little tomato sauce (about 1–2 tablespoons) into the bottom of each ramekin and spread to coat.
Place a single layer of 2–3 cooked raviolis (depending on size) into each ramekin.
Divide 1/3 cup of sauce among the ramekins, pouring over the ravioli and spreading with the back of a spoon, then sprinkle each with about 1/3 cup shredded mozzarella.
Repeat the layering: add another layer of ravioli, 1/3 cup sauce, and 1/3 cup mozzarella to each ramekin.
Top each ramekin with the remaining ravioli, divide the remaining tomato sauce among them, then finish with the remaining shredded mozzarella and a sprinkle of grated Parmesan.
Bake in the preheated oven for 20–25 minutes, until the cheese is melted and bubbling.
Let the individual lasagnas cool for about 5 minutes before serving; ramekins will be very hot.