Preparation of Ingredients: Assemble all the required ingredients to ensure a streamlined cooking process.
Boiling the Onions: In a medium-sized pot, bring four cups of water to a rolling boil. Introduce the peeled pearl onions and boil until they are tender. Upon completion, drain the onions using a colander, ensuring to reserve two cups of the cooking liquid for later use. Set the cooked onions aside.
Roux Creation: Place a saucepan over medium heat and melt six tablespoons of butter. Once melted, briskly whisk in an equal amount of all-purpose flour to form a smooth paste. This mixture, known as a roux, will serve as the thickening base for the cream sauce.
Sauce Preparation: Gradually add the three cups of milk to the reserved onion cooking liquid, then slowly incorporate this liquid into the roux while continuously stirring to prevent any lumps from forming. Continue to cook the mixture, stirring consistently, until it thickens into a rich cream sauce.
Final Seasoning and Assembly: Season the cream sauce with a pinch of salt and freshly ground black pepper according to taste. Gently fold in the cooked pearl onions, ensuring they are evenly coated with the sauce. Transfer the completed dish to a suitable serving vessel.