Bring a large pot of salted water to a boil over high heat. Add the medium pasta shells and cook until just under al dente, about 8 minutes.

In a large skillet, heat 1 tablespoon of olive oil over medium-high heat. Add 1 pound of ground beef and cook until browned, about 5-7 minutes.

In the same skillet, add another tablespoon of olive oil if necessary, and toss in a small diced onion. Sauté for about 2 minutes until slightly softened, then add 2 minced garlic cloves.

Sprinkle in 2 tablespoons of all-purpose flour, whisking continuously until it lightly browns. Gradually add 1 cup of beef stock and 1 cup of marinara sauce.

Once the sauce has thickened, stir in the cooked pasta and the browned beef, then mix in 1 cup of heavy cream.

Remove from heat and stir in ½ cup of sour cream, followed by 1 cup of shredded cheddar cheese.