Preheat your oven to 425°F (220°C).

Steam fresh spinach in a saucepan over simmering water for about 1 minute until wilted. Chop finely and set aside.

Bring a large pot of salted water to a boil. Cook the jumbo pasta shells for about 10 minutes until al dente, drain, and drizzle with olive oil to prevent sticking.

In a medium bowl, combine chopped spinach, ricotta cheese, pecorino cheese, minced garlic, oregano, lemon zest, red pepper flakes, salt, and black pepper. Mix thoroughly.

Spread marinara sauce evenly across the bottom of a 9x13 inch baking dish.

Fill each cooked pasta shell with the spinach and cheese mixture and place in the baking dish seam-side up.

Cover the dish with foil and bake for about 20 minutes.

Remove from oven, let sit for a few minutes, and serve hot with extra marinara sauce.