Preheat the oven to 375°F (190°C). Cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside.

In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes. Stir in the minced garlic and cook for an additional minute.

Add the chopped lobster meat and spinach to the skillet. Cook until the spinach is wilted, about 2-3 minutes. Remove from heat and let cool slightly.

In a large bowl, combine the ricotta cheese, half of the mozzarella cheese, Parmesan cheese, salt, black pepper, and red pepper flakes (if using). Stir in the lobster and spinach mixture until well combined.

Spread half of the marinara sauce evenly in the bottom of a 9x13 inch baking dish. Stuff each cooked pasta shell with the lobster and cheese mixture and place them in the baking dish.

Pour the remaining marinara sauce over the stuffed shells and sprinkle with the remaining mozzarella cheese.

Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.

Remove from the oven and let cool for a few minutes. Garnish with fresh parsley before serving.