Preheat the oven to 350 degrees F (175 degrees C).

Bring a large pot of salted water to a boil. Gently place pasta shells in boiling water; return water to a boil. Cook until shells are just tender; drain well.

Squeeze spinach dry and place in a large mixing bowl. Add ricotta, 1/3 cup Parmesan cheese, fennel, basil, and garlic. Season with salt and pepper; mix well.

Spread 1/2 cup spaghetti sauce evenly over the bottom of a 9x13-inch baking dish.

Fill each pasta shell with spinach-cheese mixture. Arrange shells, filling-side up, in the baking dish. Spoon remaining spaghetti sauce over shells. Sprinkle remaining Parmesan cheese on top of shells.

Cover the pan loosely with aluminum foil and bake in the preheated oven until heated through, about 30 minutes.