Bring a large pot of salted water to a rolling boil. Add the pasta shells and cook for 2 minutes less than the package directions indicate (they should still be slightly firm). Drain well and set aside.
While pasta cooks, heat olive oil in a large skillet over medium-high heat. Add ground beef and cook for 4-6 minutes, breaking it apart with a wooden spoon, until deeply browned. Drain excess fat, leaving about 1 tablespoon in the pan. Transfer beef to a plate.
In the same skillet over medium heat, add diced onion and cook for 3 minutes, stirring often, until softened and translucent. Add minced garlic and cook for 1 minute until fragrant.
Sprinkle flour over the onion mixture and whisk constantly for 1 minute until the flour turns slightly golden and smells toasty.
Slowly pour in the beef stock while whisking continuously to prevent lumps. The mixture will thicken. Add marinara sauce, Italian seasoning, dried parsley, oregano, and smoked paprika. Stir well to combine.
Bring the sauce to a gentle boil, then reduce heat to medium-low. Simmer for 7-9 minutes, stirring occasionally, until the sauce has reduced and thickened enough to coat the back of a spoon.
Return the cooked beef to the skillet along with the drained pasta shells. Stir gently to combine everything evenly.
Pour in the heavy cream and stir until heated through, about 2 minutes. Taste and season with salt and black pepper as needed.
Remove from heat and fold in the sour cream until smooth and creamy.
Add the shredded cheddar cheese and stir gently until melted and silky, about 1-2 minutes. Serve immediately while hot, garnished with fresh parsley if desired.
