Bring a large pot of salted water to a boil. Cook medium pasta shells according to package directions until al dente, about 8-10 minutes. Reserve 1/2 cup of pasta water before draining. Drain shells thoroughly and set aside.

Heat olive oil in a large, deep skillet over medium-high heat. Add ground beef and cook for 3-5 minutes, breaking it into small crumbles with a wooden spoon, until browned with no pink remaining. Drain excess grease if needed, leaving 1-2 tablespoons for flavor.

Add diced sweet onion to the skillet with the beef. Cook for about 2 minutes, stirring occasionally, until softened and translucent. Add minced garlic and cook for 1 minute more, stirring constantly until fragrant.

Sprinkle flour over the beef and onion mixture, stirring continuously to coat everything evenly. Cook for about 1 minute, stirring constantly, until flour turns lightly golden.

Gradually pour in beef stock, stirring constantly to prevent lumps. Add marinara sauce, Italian seasoning, dried parsley, dried oregano, and smoked paprika. Stir until well combined. Increase heat to bring to a gentle boil, then reduce to medium-low and simmer for 6-8 minutes, stirring occasionally, until sauce thickens slightly.

Add drained pasta shells back to the skillet and stir to combine. Pour in heavy cream and stir everything together. Heat for 1-2 minutes until warmed through.

Reduce heat to low. Stir in sour cream until fully incorporated. Add shredded cheddar cheese and fold in gently, stirring until cheese melts completely. If sauce is too thick, add reserved pasta water a tablespoon at a time. Season with salt and pepper to taste.

Transfer to serving bowls and serve hot. Garnish with fresh parsley, extra cheese, or red pepper flakes if desired.