Preheat oven to 375°F (190°C). Spray a 9x13 inch baking dish with non-stick cooking spray and spread a small amount (about 1/2 cup) of marinara sauce (if using) onto the bottom. Alternatively, use a drizzle of the sun-dried tomato oil.

Cook the jumbo shells in a large pot of salted boiling water according to package directions, removing them 2-3 minutes before fully al dente. Rinse with cold water to stop cooking and cool.

In a large bowl, make the filling. Combine ricotta, squeezed-dry spinach, 1 cup of mozzarella, 1/2 cup of parmesan, beaten egg, minced garlic, garlic powder, onion powder, red pepper flakes, salt, and black pepper. Stir well.

Gently fold in half (1/2 cup) of the chopped sun-dried tomatoes and the reserved 1 tablespoon of sun-dried tomato oil into the filling.

Stuff each cooled shell and place them, side-by-side and slightly overlapping, in the prepared baking dish.

Pour the remaining marinara sauce (if using) over the shells, lightly covering them. Skip this step for non-red sauce base and focus on the topping.

Scatter the remaining 1/2 cup of sun-dried tomatoes over the top.

Sprinkle with the remaining 1 cup of mozzarella and 1/2 cup of parmesan cheese.

Cover the dish tightly with foil and bake for 25 minutes.

Remove the foil and continue to bake for another 10-15 minutes, or until the cheese is melted, bubbly, and slightly golden. A shell should be over 160°F internal.

Let rest for 5-10 minutes before serving. Garnish with fresh chopped parsley if desired.