olive or vegetable oil for frying
Chop the leeks and fry gently in a small amount of oil for about 5 minutes until softened.
Gently melt the margarine in a saucepan, add the soya milk and sieve in the flour. Whisk in the flour until there are no lumps, then increase the heat and keep whisking until the mixture thickens. Turn the heat down and add the rest of the sauce ingredients. Add leeks to sauce and serve.