Preheat your oven to 375°F. Bring a large pot of water to a rolling boil. Once boiling, generously salt the water with 2 teaspoons of salt, then add the pasta shells. Cook the shells for about 8 minutes, just until al dente. Turn off the heat, drain, and set aside. Be sure to reserve ½ cup of the pasta water before draining.
In a large skillet or stovetop-safe casserole dish, add the ground beef and Italian sausage. Cook over medium heat, breaking the meat apart as it browns. Season with seasoning salt, black pepper, and crushed red pepper flakes. Stir well to evenly distribute the seasoning. Once the meat is fully browned, remove it from the dish and set it aside, leaving the rendered fat behind.
Add the diced peppers and onions to the same dish and cook until softened and translucent. Next, stir in the mushrooms and garlic, cooking for another 2–3 minutes until fragrant.
Pour in the tomato sauce and fire-roasted diced tomatoes, stirring to combine. Add the fresh herbs and give everything a good stir. Pour in the reserved pasta water, then reduce the heat to low-medium. Let the sauce simmer for about 15 minutes so the flavors can come together.
Scoop out about 1 cup of the sauce and add it to a large bowl along with the cooked meat. Stir in the ricotta and Parmesan cheese until well combined.
Now it’s time to stuff the shells. Fill each shell generously with the meat and cheese mixture and set them aside. Scoop out another cup of sauce and set that aside as well, leaving the remaining sauce in the dish.
Carefully arrange the stuffed shells directly into the dish with the sauce. Spoon the reserved sauce evenly over the shells, making sure they’re well coated. Layer the sliced mozzarella evenly over the top.
Cover the dish tightly with foil or a lid and bake for 40 minutes. Remove the foil and continue baking uncovered for an additional 20 minutes, or until the cheese is melted and lightly golden.
Remove from the oven and sprinkle with fresh parsley. Let the dish rest for 15–20 minutes before serving. Serve warm with crusty garlic bread and a fresh salad on the side.