Cook the jumbo pasta shells in a large pot of salted boiling water until al dente, about 8-10 minutes. Drain and rinse with cold water to stop the cooking process.

In a large skillet, heat 1 tbsp of olive oil over medium heat. Add 3 minced garlic cloves and sauté for about 30 seconds until fragrant. Add the chopped shrimp and cook until pink and opaque, about 3-4 minutes. Remove from heat and let cool slightly.

In a large mixing bowl, combine the cooked shrimp, ricotta cheese, 1 cup of mozzarella, grated Parmesan, egg, parsley, Italian seasoning, salt, and pepper. Mix well until all ingredients are combined.

Take each cooked shell and fill with about 2 tablespoons of the shrimp mixture. Place each filled shell in a greased baking dish.

In the same skillet, melt 3 tbsp of unsalted butter over medium heat. Add 3 minced garlic cloves and cook for 30 seconds. Whisk in 2 tbsp of flour, cooking for 1 minute. Gradually stir in the half-and-half and chicken broth, continuing to whisk until smooth. Add 1 cup of Parmesan, seasoning, and red pepper flakes, if desired, and cook until thickened, about 2-3 minutes.

Pour the sauce over the stuffed shells, ensuring each is covered. Sprinkle remaining mozzarella and Parmesan cheese over the top.

Bake in a preheated oven at 350°F (175°C) for 15-20 minutes until bubbly and golden.

Once out of the oven, let the dish sit for 5 minutes. Garnish with additional chopped parsley before serving.