Classic Croque Monsieur

Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper.

Combine milk and cream in a microwave-safe container and microwave on high power, about 90 seconds. The liquid should be warm to the touch.

Melt butter in a small saucepan over medium heat. Stir in flour and keep stirring until all the flour is coated in butter, and the flour and butter begin to bubble, about 2 minutes. Gradually stir in warmed milk and cream and cook until sauce begins to thicken, stirring constantly. Continue stirring, until the sauce is thick and bubbly, 3 to 5 minutes. Remove from heat and season with salt, pepper, and nutmeg. Set aside.