Cook the pasta shells: Fill a large pot with water, add a generous amount of salt, and bring it to a vigorous boil. Once boiling, add the jumbo pasta shells and cook them until they reach an al dente texture, following the instructions on the package. Afterward, drain the shells and let them cool before using.

Cook the sausage: In a spacious skillet set to medium heat, brown the Italian sausage, using a spoon to crumble it as it cooks. Allow it to sizzle for approximately 5 to 7 minutes, or until it's thoroughly browned. Stir in the minced garlic and continue cooking for another minute until the aroma fills the kitchen. Stir in the can of diced tomatoes and let the mixture simmer for a few minutes. Season with salt and pepper to taste. Remove from heat and set aside to cool slightly.

Prepare the filling: In a large mixing bowl, combine the cooked sausage mixture, thawed and squeezed-dry spinach, ricotta cheese, and shredded mozzarella. Mix thoroughly until all the ingredients are evenly blended. Sample the mixture and modify the seasoning with salt and pepper as necessary.

Preheat the oven: Preheat your oven to 375°F (190°C).

Stuff the shells: Pour 1 cup of marinara sauce into the bottom of a 9x13-inch baking dish. Fill each cooked pasta shell with a generous spoonful of the sausage, spinach, and cheese mixture. Arrange the stuffed shells in the baking dish with the openings facing upward. Once all the shells are filled and arranged, pour the remaining marinara sauce over the top.

Bake the shells: Seal the baking dish with aluminum foil and place it in the preheated oven to bake for 20 minutes. Remove the foil and bake for an additional 10 minutes, or until the sauce is bubbling and the cheese on top is melted and golden.

Serve: Remove the stuffed shells from the oven and let them cool slightly before serving. Garnish with additional mozzarella or Parmesan cheese if desired. Serve hot and enjoy!