Step 3: Make the Cheese Sauce
In a medium saucepan, melt butter over medium heat.
Whisk in flour and cook for 1-2 minutes, stirring constantly, until golden and bubbly. This cooks out the raw flour taste.
Slowly whisk in warm milk (and cream, if using) until smooth. Continue whisking until the sauce thickens, 3-5 minutes.
Remove from heat. Stir in Dijon mustard, garlic powder, onion powder, nutmeg, salt, and pepper.
Gradually add cheddar and Parmesan, stirring until completely melted and smooth.
Taste and adjust seasoning. The sauce should be rich, savory, and just slightly sharp.
