Melt butter in a large pot over low heat.

Add chopped white onions, green bell peppers, and celery stalks to the pot.

Saute the vegetables until they are tender but not browned, stirring occasionally.

Pour milk into the pot with the sauteed vegetables.

Add bay leaves to the milk and vegetable mixture.

Bring the mixture to a simmer over low heat.

Gradually whisk in the roux to the simmering sauce.

Continue to cook the sauce, stirring constantly, until it reaches the desired thickness.

Remove the pot from the heat.

Season the sauce to taste with salt and pepper.

Strain the sauce through a fine-mesh sieve to remove any lumps or solids.

Stir in cream to the strained sauce.

Serve immediately or keep warm until ready to use.