Boil Pasta: Bring a large pot of generously salted water to a boil. Add the pasta shells and cook according to package directions until al dente. Drain, reserving 1/4 cup of pasta water, and set aside.

Cook Beef: In a large skillet or Dutch oven over medium-high heat, add the ground beef. Cook, breaking the meat apart, for 5-7 minutes until completely browned. Drain excess grease, leaving about 1 tablespoon in the pan.

Sauté Aromatics: Reduce heat to medium. Add the diced onion to the beef and cook for 3-4 minutes until soft. Stir in the minced garlic and cook for 1 minute until fragrant.

Simmer: Pour in the marinara sauce, Italian seasoning, salt, pepper, and red pepper flakes. Stir to combine. Reduce heat to low and let the sauce simmer for 5 minutes.

Combine: Gently stir the cooked pasta shells into the meat sauce until fully coated. If the sauce is too thick, add a splash of the reserved pasta water.

Melt Cheese: Remove the skillet from the heat. Sprinkle the grated cheese evenly over the top. Cover the pan with a lid for 2-3 minutes until the cheese is completely melted.

Serve: Garnish with fresh chopped parsley and serve immediately hot.