Preheat oven to 350°F (175°C).

Cook medium shells according to package directions; drain well.

In a baking dish, create the first layer with spaghetti sauce.

Add a layer of cooked shells over the sauce.

Spread sour cream evenly over the shell layer.

Place slices of Provolone cheese over the sour cream.

Repeat the layering process: sauce, shells, sour cream, Provolone.

For the last layer, arrange slices of Mozzarella cheese.

Cover the baking dish with a lid or aluminum foil.

Bake in the preheated oven for 45 minutes to 1 hour, or until the cheese is melted and bubbly.