Cook pasta shells in salted water according to package instructions until al dente. Drain and set aside.
Heat olive oil in a large skillet over medium-high heat. Add ground beef and cook until browned. Drain excess fat if needed.
Add diced onion and cook until softened, about 3–4 minutes. Stir in garlic and cook for 30 seconds.
Stir in tomato paste and cook for 1 minute to deepen flavor.
Pour in heavy cream and milk. Stir well and simmer gently until slightly thickened.
Reduce heat to low and stir in shredded cheese until melted and smooth.
Add cooked pasta shells and toss until evenly coated in the sauce.
Taste and adjust seasoning. Serve hot.