Bring a large pot of salted water to a boil. Add the pasta shells and cook according to package directions until al dente. Drain well and set aside.
While the pasta is cooking, heat olive oil in a large skillet or Dutch oven over medium-high heat. Add the ground beef and cook until browned, breaking it apart with a spoon (about 3–5 minutes).
Stir in the diced onion and cook for another 2–3 minutes, until softened. Add the minced garlic and Italian seasoning, sautéing for about 1 minute until fragrant.
Sprinkle the flour over the beef mixture and stir well to coat. Cook for 1 minute to eliminate any raw flour taste.
Pour in the beef stock, scraping any flavorful bits from the bottom of the pan. Add the tomato sauce, then stir to combine. Reduce the heat to medium and simmer the sauce for about 5 minutes, until slightly thickened.
Lower the heat and pour in the heavy cream. Stir gently until fully combined and the sauce turns luxuriously creamy.
Add the cooked pasta shells into the sauce, stirring to coat every piece evenly.
Remove from the heat and sprinkle in the shredded extra-sharp cheddar. Stir until the cheese is melted and the sauce is velvety. Season with kosher salt and freshly ground black pepper to taste.
Serve hot, garnishing with extra cheese, fresh herbs, or a twist of cracked pepper if desired.
Notes
Store leftovers in an airtight container in the refrigerator for up to 3–4 days. To reheat, microwave individual portions with a splash of water or milk, stirring halfway through for even heating.