Bring a large pot of heavily salted water to a rolling boil. Cook jumbo shells according to package directions, but undercook by 1-2 minutes until al dente. Drain and rinse with cool water. Meanwhile, heat olive oil in a large skillet over medium-high heat. Add the Italian sausage and cook until browned and crumbled. Drain excess grease.

In a large bowl, combine the whole milk ricotta, cooked sausage, 1/4 cup (25g) of Parmesan, chopped parsley, egg, garlic powder, onion powder, salt, and pepper. Stir until just combined.

Preheat oven to 375°F (190°C). Spread 2 cups of marinara sauce in the bottom of a 9x13-inch baking dish. Stuff each cooked shell with the ricotta mixture and arrange them in the dish. Top with the remaining marinara.

Sprinkle with shredded mozzarella and the remaining 1/4 cup (25g) of Parmesan. Cover with foil and bake for 20 minutes. Remove foil and bake for another 10-15 minutes, until the cheese is melted, bubbly, and golden-brown. Let rest for 5-10 minutes, then garnish with fresh parsley and serve.