Melt 2 tbsp butter in a saucepan over medium-low heat
Add 2–3 cloves minced garlic and cook 30–60 seconds
Sprinkle in 2 tbsp all-purpose flour (optional for thicker sauce) and whisk 1 minute
Slowly whisk in 1 to 1½ cups heavy cream
Simmer gently 3–5 minutes, whisking occasionally
Reduce heat to low
Add 1½ to 2 cups grated Parmesan (or 1 cup Parmesan + 1 cup Pecorino Romano) and whisk until smooth
Season with ½ tsp salt and ¼ tsp black pepper (adjust to taste)
Stir in 1–2 tbsp pasta water or additional cream to reach desired consistency
Turn off heat and stir in 1 tbsp chopped parsley (optional)