In a saucepan, melt the butter and fry the leeks and garlic until soft.
Stir in the flour until a roux is formed and slowly whisk in the milk. I only add 2 cups milk at the beginning to ensure that the sauce isn’t too thin. You want the sauce to be thick enough to not ooze off the toast completely.
Lower the heat and allow the sauce to cook gently until the ‘floury’ taste has disappeared. Add the seasonings and the cheddar and take off the heat. Stir until the cheese has melted and adjust the seasoning if necessary.
Heat the broiler/grill of the oven and toast the bread until golden brown on both sides.
Top with a few spoonfuls of the sauce and place back into the oven. Cook until golden and bubbling then remove from the oven and serve.