Begin by boiling a large pot of salted water over high heat. Once boiling, add the jumbo pasta shells and cook them according to package instructions until al dente, usually around 9-11 minutes. Drain the shells carefully and set them aside on a baking sheet to cool slightly so they don't stick together.
In a mixing bowl, combine ricotta cheese, finely chopped sun-dried tomatoes, grated Parmesan cheese, half of the shredded mozzarella cheese, beaten egg, minced garlic, Italian seasoning, salt, black pepper, and optional red pepper flakes. Mix well until all ingredients are fully incorporated into a creamy filling.
Preheat your oven to 375°F (190°C). While it's heating up, you can start assembling your stuffed shells which will save time once everything is ready.
Pour about one cup of marinara sauce onto the bottom of a baking dish to prevent sticking. Carefully take each cooked shell and fill it generously with the ricotta mixture using a spoon or piping bag if desired. Place each filled shell seam-side up in the baking dish.
Once all shells are filled and arranged in the baking dish, pour remaining marinara sauce over them evenly ensuring they are well covered but not drowning in sauce. Sprinkle remaining shredded mozzarella on top along with some extra grated Parmesan cheese for added cheesiness.
Cover the baking dish with foil and bake in your preheated oven for about 20 minutes. Afterward, remove foil and bake an additional 10 minutes or until cheese is bubbly and golden brown on top. Remove from oven, garnish with fresh basil leaves before serving warm alongside extra marinara if desired!