Bring a large pot of salted water to a boil. Cook the pasta shells according to package directions until al dente. Drain well and set aside.
Heat the olive oil in a large skillet over medium-high heat. Add the ground beef and cook for 3–5 minutes, breaking it into small crumbles as it browns. Cook ground beef thoroughly to 160°F (71°C).
Drain excess fat if necessary, transfer the browned beef to a plate, and set aside.
Add the diced onion to the same skillet. Cook for about 2–3 minutes, stirring frequently, until it begins to soften.
Add the minced garlic and cook for approximately 30–60 seconds until fragrant.
Sprinkle in the flour and stir continuously for about 1 minute to remove its raw flavor.
Slowly pour in the beef stock while whisking or stirring to prevent lumps.
Add the marinara sauce, Italian seasoning, dried parsley, oregano, and smoked paprika. Stir until thoroughly combined.
Bring the sauce to a gentle boil, then reduce the heat and simmer for 6–8 minutes, stirring occasionally, until slightly thickened.
Return the cooked ground beef to the skillet and add the drained pasta. Stir until the shells are evenly coated.
Pour the heavy cream into the Creamy Beef and Shells and cook over low heat for 1–2 minutes until everything is hot and creamy. Season with salt and black pepper to taste.
Reduce the heat to low and stir in the sour cream.
Add the freshly grated cheddar gradually, stirring until completely melted into the sauce.
Remove from the heat and serve immediately. Garnish with fresh parsley if desired.