Simmer the pearl onions in water for 5 minutes, drain them, and set them aside.

Melt the butter in a saucepan over medium heat and whisk in the flour for 1 minute without letting it brown.

Add the warm milk gradually, whisking constantly, and simmer for 4 minutes until the sauce lightly coats a spoon.

Stir in the salt, pepper, nutmeg, onions and frozen peas.

Cook for 4 to 5 minutes until the peas are hot and bright green, thinning with 2 tablespoons milk only if the sauce becomes pasty.