Start with the pasta. Cook shells in heavily salted water for 2 minutes less than package directions—they should be barely pliable, not fully tender. This is critical. Overcooked shells = mushy disaster. Drain and spread on a baking sheet to prevent sticking.

Build your filling. Mix ricotta, squeezed spinach (really squeeze it—water is the enemy here), half the mozzarella, most of the Parmesan (save 2 tablespoons), garlic, egg, basil, oregano, and red pepper flakes. Season aggressively with salt and pepper. The filling should taste slightly overseasoned when raw—it mellows during baking.

Prep your vessel. Spread 1½ cups marinara across the bottom of a 9x13 baking dish. Don't skip this base layer—it prevents scorching and creates that perfect saucy bottom that everyone fights over.

The assembly. Fill each shell with about 2 tablespoons of ricotta mixture—enough to mound slightly but not overflow. Nestle them seam-side up in the dish. They should fit snugly but not be crammed. Pour remaining marinara over and around (not directly on top of) the shells.

The cheese finish. Scatter remaining mozzarella and Parmesan over everything. Cover tightly with foil (spray the underside with oil to prevent sticking).

The bake. 25 minutes covered at 375°F, then 10 minutes uncovered. You're looking for aggressive bubbling around the edges and golden spots on the exposed cheese. Let it rest for 10 minutes before serving—patience here prevents molten cheese burns.