Cook the pasta. Bring a large pot of water to a boil. Season the water generously with salt. Cook the shells 1 to 2 minutes under the recommended cooking time so the shells are just slightly more than al dente. Drain and transfer to a parchment lined sheet tray. Let cool.
Make the stuffing. In a medium bowl, combine the ricotta, parmesan, mozzarella, egg, garlic, and seasoning. Preheat the oven to 350°F.
Stuff the shells. Using a piping bag or spoon, fill the shells with the ricotta mixture. A piping bag or plastic bag is highly recommended. Just cut the tip off the bag and squeeze into each shell.
Make the sauce. Heat a large skillet over medium-high heat, add the olive oil and let it get hot. Brown the sausage in the hot oil, breaking it up into small pieces as it cooks. Add the chopped kale or spinach and cook, stirring frequently. Once the greens have started to wilt, pour in the white wine, scraping any cooked on meat from the bottom of the pan. Stir the jar of marinara sauce into the pan. If opting for a creamier red sauce, add in the heavy cream and butter, stirring until melted and incorporated.
Bake the shells. Remove about ¾ cup to 1 full cup of the sausage marinara. If the skillet is large enough and oven safe, arrange the shells over the remaining sauce or transfer the sauce to a 9x13 baking dish before covering with the stuffed shells. Top with reserved saice and mozzarella. Bake at 350°F for 25 to 30 minutes until the ricotta is thick and creamy (but not runny) and the cheese is melted. Serve right away or cover and reheat before serving.