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, reducing the cook time by 1 minute; drain and set aside.
Set out four oven-safe ramekins (about 5 inches wide and 3 inches deep) and spoon 1–2 tablespoons of tomato sauce into the bottom of each; spread lightly.
Place a single layer of 2–3 raviolis in each ramekin (depending on ravioli size).
Divide 1/3 cup of sauce among the ramekins, spooning it over the ravioli, then sprinkle each with about 1/3 cup shredded mozzarella.
Repeat the layering once more: another layer of ravioli, 1/3 cup sauce, and 1/3 cup mozzarella.
Arrange any remaining ravioli on top of each dish, divide the rest of the sauce evenly among the ramekins, and top with the remaining mozzarella and the 1/2 cup grated Parmesan.
Place the ramekins on a baking sheet and bake for 20–25 minutes, until the cheese is melted and bubbling.
Remove from the oven and let cool about 5 minutes before serving; ramekins will be very hot.
