Bring a large pot of salted water to a boil. Add fettuccine and cook until al dente (check the package for timing—usually 8–10 minutes). Reserve 1 cup of pasta water before draining. Toss the drained pasta with a drizzle of olive oil to prevent sticking.

Heat 2 tbsp butter in a skillet over medium-high heat. Add seafood in a single layer and cook for 2–3 minutes per side (shrimp should turn pink; scallops golden). Transfer seafood to a plate and cover to keep warm.

Reduce heat to medium. Add remaining butter and garlic to the skillet. Cook until fragrant (about 30 seconds), then pour in the heavy cream. Simmer gently for 2 minutes. Gradually stir in Parmesan until the sauce is smooth. If it’s too thick, add reserved pasta water 1 tbsp at a time. Season with salt, pepper, and red pepper flakes (if using).

Add the cooked pasta and seafood to the skillet. Toss gently to coat in the sauce. Remove from heat and sprinkle with fresh parsley.