Preheat oven to 375°F. In a large skillet over medium-high heat, cook onion in olive oil until softened, 5 minutes. Add garlic and cook 1 minute. Add ground beef, 1 tsp salt, pepper, and Italian seasoning. Cook, breaking up meat, until browned, about 8 minutes. Stir in marinara, simmer while you prepare the filling.

In a medium bowl, mix ricotta, squeezed spinach, egg, and ¼ cup Parmesan until combined. Taste and add a pinch of salt if needed.

Spread ½ cup sauce in a 9x13-inch dish. Layer 3 sheets, half the ricotta mixture, a third of the mozzarella, and about 1 cup sauce. Repeat with 3 sheets, remaining ricotta, another third of mozzarella, and more sauce. Top with last 3 sheets, remaining sauce, remaining mozzarella, and remaining ¼ cup Parmesan.

Cover loosely with foil and bake 25 minutes. Remove foil and bake 20–25 minutes more, until top is golden and edges are bubbling.

Let rest 10–15 minutes, then slice and serve warm.