Melt 4 Tbsp. unsalted butter in a medium saucepan over medium heat.
Stir in ¼ cup (32 g) all-purpose flour and cook, stirring constantly, until the paste cooks and bubbles but does not brown, about 2 minutes.
Slowly add 2½ cups whole milk while whisking to incorporate.
Bring mixture to a simmer, stirring constantly, then reduce burner to low heat and cook until sauce coats the back of a spoon, 2–3 minutes more.
Remove from heat and stir in ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, a pinch of freshly ground black pepper or white pepper, a pinch of freshly ground nutmeg.
Taste and add additional kosher salt as needed.
