Bring a large pot of water to a boil. Add the jumbo shells and cook them al dente, following the package instructions so they remain firm but tender. Once cooked, drain the shells and refill the pot with cold water. Place the cooked shells back into the cold water to stop the cooking process and prevent sticking. Set them aside until ready to use.

While the shells cool, heat the olive oil in a large skillet over medium heat. Add the finely chopped onion and crushed garlic, sautéing until the onion is softened, about 5 minutes. Add the ground beef to the pan, breaking it up and cooking until browned and no longer pink. Season the mixture with salt and pepper. If using, pour in the red wine and simmer until most of the liquid has evaporated, about 3 minutes. Stir in 1/2 cup of marinara sauce and the chopped spinach. Cook for about 3 minutes, until the spinach is wilted. Remove the pan from the heat and let the mixture cool slightly.

In a large mixing bowl, combine the ricotta cheese, 1 cup shredded mozzarella, eggs, and chopped parsley. Mix until everything is evenly combined. Add the slightly cooled meat and spinach mixture from Step 2, and stir again until fully incorporated. This creamy and hearty filling is what gives the stuffed shells their delicious flavor!

Preheat your oven to 375°F (190°C). Pour 1/2 cup of marinara sauce into the bottom of a 9x13-inch baking dish and spread it evenly. Drain the shells from the cold water. Carefully fill each shell with about 2 rounded tablespoons of the cheese and meat filling (an ice cream scoop works great for this). Arrange the stuffed shells in the prepared baking dish, packed snugly together. Pour the remaining marinara sauce evenly over the top of the shells.

Sprinkle the remaining 1 cup of shredded mozzarella cheese over the stuffed shells. Cover the baking dish with foil (or a lid) and bake for about 40 minutes, until hot and bubbly. Remove the foil and bake for another 3-5 minutes, or until the cheese turns golden. I like to let the shells rest for a few minutes before serving—this helps the flavors come together and makes them easier to serve. Enjoy your homemade stuffed shells!