1/4 onion (with 1 bay leaf stuck to it using 2 whole cloves)
1 pinch freshly grated nutmeg (optional)
Simmer gently for 15 minutes, uncovered, to infuse flavor into the milk. Discard the onion, the bay leaf, and the cloves. Meanwhile, melt in a medium saucepan over low heat:
2 tablespoons unsalted butter
Stir in:
2 tablespoons all-purpose flour
Cook, uncovered, stirring occasionally with a wooden spoon or spatula, over medium low heat until the roux is just fragrant but not darkened, 2 to 3 minutes. Remove from the heat and let cool slightly. Slowly whisk in the warm milk and return the saucepan to the heat. Bring the sauce slowly to a simmer, whisking to prevent lumps, and cook, stirring often and skimming any skin that forms on the surface, over low heat, without boiling, until it reaches the consistency of thick cream soup, 8 to 10 minutes. Strain though a fine-mesh sieve, if desired. Stir in:
2 to 4 tablespoons Dijon mustard, grainy French mustard, or brown English mustard to taste
