3. Cook bacon in a dash of oil in a small frying pan. Set aside. Melt the butter in the saucepan and gently cook the leek until softened. Add the flour and stir. Gradually add the milk stirring continuously over a medium heat until the sauce thickens and boils. Reduce heat and simmer a further minute. Remove from the heat and stir in the grated tasty cheese and cooked bacon. Season to taste. Pour the sauce over the vegetables.