Bring a large ovenproof pot of salted water to a boil over high heat. Once the water reaches a rapid boil, add the pasta and stir to prevent sticking. Cook for only half of the recommended time—approx. 4-5 minutes. Drain the pasta in a colander then set aside.
Preheat oven to 350 degrees and move a rack to the lower 1/3 of the oven—make sure there is enough clearance for the pot!
Meanwhile, place the same pot over medium-high heat and add the ground beef. Season with salt and cook, breaking it up into chunks until mostly browned. Add the diced onion, bell pepper and mushrooms and continue cooking an additional 5–6 minutes until the onion and bell pepper have begun to soften.
Add the marinara sauce, sugar, dried basil, crushed red pepper flakes and tomatoes (with juice) and stir to combine. Bring liquid to a bubble then reduce heat to medium-low. Simmer 8 minutes then remove from heat.
Stir in sour cream, provolone, mozzarella and partially cooked pasta shells. Combine thoroughly and top with Parmesan. Cover with lid or foil and bake 30-35 minutes, until bubbling. Remove from oven and garnish with chopped parsley. Enjoy!
Don’t worry if the sauce mixture seems a little bit thin. The pasta is only half cooked and it’ll absorb quite a bit of the moisture.