1. Cook linguine pasta according to package instructions until al dente. Reserve about ½ cup pasta water before draining; set pasta aside.
2. In a large skillet over medium heat, melt 1 tablespoon butter or heat oil, and sauté diced onion until soft. Add ground beef and cook, breaking apart, until browned. Season with salt, pepper, Italian seasoning, and smoked paprika. Drain excess grease and remove beef from the skillet.
3. In the same skillet, melt remaining butter over medium-low heat. Add minced garlic and red pepper flakes if using; sauté for about 1 minute until fragrant but not browned.
4. Pour in heavy cream (and beef broth if using) and stir constantly. Simmer for 2–3 minutes until sauce thickens slightly. Gradually add reserved pasta water if needed to adjust the sauce consistency.
5. Stir in mozzarella and Parmesan cheeses until melted and smooth, stirring continuously to avoid curdling.
6. Return cooked ground beef to the skillet, then add the drained pasta. Toss everything together to coat evenly with the creamy sauce. Cook for another 2–3 minutes.
7. Remove from heat and garnish with chopped fresh parsley before serving.