Preheat your oven to 425°F (220°C).

Steam about 2 cups of fresh spinach for 1 minute until wilted, then chop it finely.

Boil a large pot of salted water, add 20 large pasta shells, and cook for about 10 minutes until al dente.

In a mixing bowl, combine chopped spinach, 1 cup ricotta, 0.5 cup pecorino cheese, 2 minced garlic cloves, 1 teaspoon oregano, zest of one lemon, red pepper flakes, and season with salt and pepper.

Spread a thin layer of marinara sauce across the bottom of a 9x13 baking dish.

Stuff each cooked shell with the spinach and ricotta filling; place them seam-side up in the baking dish.

Cover the baking dish with aluminum foil and bake for 20 minutes.

Remove the foil and let the shells rest for a few minutes before serving.