Melt the butter in a small saucepan over medium heat. Add the garlic and cook for 1 to 2 minutes, until fragrant but not browned.
Stir in the flour and cook for 1 minute, stirring constantly, to make a smooth paste.
Slowly whisk in the milk, pouring a little at a time to avoid lumps. Bring to a gentle simmer and cook for 2 to 3 minutes, until slightly thickened.
Turn off the heat. Stir in the parmesan, salt, pepper, and chili flakes (if using). Add lemon juice, 1 tsp at a time, to brighten the flavor if desired.
Notes
Serving – This recipe makes about 1 1/2 cups of sauce, which is enough for 8 oz dry pasta or 2 lbs of chicken wings. Parmesan matters – Use finely grated cheese so it melts smoothly into the sauce. Pre-shredded often clumps.Too thick? – If the sauce thickens too much, whisk in a splash of milk before serving.Make it richer – Swap in half-and-half or cream for the milk if you want a thicker sauce.Use right away – This sauce is best fresh, but it can be cooled and reheated gently.