Combine the milk, onion slices, bay leaf, carrot slices, and peppercorns in a saucepan and bring to the boil. Simmer 10 minutes. Strain and reserve the milk, discarding the vegetables. Melt the butter in a heavy pan over medium heat. Stir in the flour and cook for a minute or so. Add the milk all at once and stir until boiling and thick, 3 to 4 minutes. Season to taste with salt and pepper as needed.
