Melt 3 Tbsp butter in a 12-inch non-stick skillet over medium heat.
Add 6 grated garlic cloves and cook for 30 seconds until fragrant, stirring constantly.
Stir in 3 Tbsp all-purpose flour and cook for 1 minute until lightly golden, stirring constantly.
Gradually add 3 cups half and half, stirring as you pour, and season with salt and pepper to taste.
Reduce heat to medium-low and cook until sauce thickens, about 1-2 minutes.
Whisk in 3 Tbsp lemon juice and 1 1/2 tsp lemon zest.
Stir in 1/2 cup grated Parmesan cheese and 1 1/2 Tbsp finely chopped parsley until cheese melts.
Serve immediately while hot.