This recipe is a flavor-sensation with pesto, garlic, lemon zest, red pepper flakes and 3 types of cheese to flavor the filling that goes into the pasta shells.
Preheat oven to 400 degrees F. Coat an 8" x 8" baking dish with non-stick spray. Cook shells in a pot of boiling water just until pliable, 8-9 minutes. Rinse shells with cold water; drain.
Cook onion in oil in a pan over medium heat until softened, 3-5 minutes. Add tomato paste, garlic, and Italian seasoning; cook until fragrant, 1 minute. Stir in tomatoes; cook until heated through, 2-3 minutes. Season sauce with salt and pepper. Cover bottom of prepared pan with a third of the sauce.
For the filling, combine pesto, 1/2 cup mozzarella, ricotta, 1/4 cup Parmesan, lemon zest and pepper flakes; season with salt. Stuff each shell with 1 Tbsp. filling and arrange in prepared pan, cheese side up. Pour remaining sauce over the shells.
Sprinkle 2 Tbsp. mozzarella and 2 Tbsp. Parmesan over top of shells. Bake until cheese gets brown, 25-30 minutes. Garnish each serving with basil.