Preheat the oven to 375°F (190°C). Lightly oil a 9×13-inch baking dish with one tablespoon of olive oil.

Bring a large pot of salted water to a boil. Cook the jumbo shells according to package directions until al dente — slightly firm as they will finish cooking in the oven. Drain and lay shells open-side up on a parchment-lined tray so they do not stick.

In a large bowl, combine ricotta, 1 cup mozzarella, Parmesan, egg, garlic, oregano, red pepper flakes, and a generous pinch of salt and pepper. Fold in the squeezed-dry spinach until evenly distributed. Taste and adjust seasoning.

Spread 1½ cups of marinara sauce across the bottom of the prepared baking dish. Drizzle the remaining tablespoon of olive oil over the sauce to prevent the shells from sticking.

Spoon the ricotta-spinach mixture into each shell, filling generously but not overflowing, and arrange them seam-side up in the dish.

Pour the remaining marinara over the stuffed shells, ensuring each shell has some sauce. Sprinkle the reserved 1/2 cup mozzarella over the top.

Cover the dish tightly with foil and bake for 20 minutes. Remove the foil and bake for another 5–10 minutes until the cheese is bubbly and starting to brown slightly.

Let the shells rest for 5 minutes before serving.