Cheese, Onion & Tomato Flan

wholemeal shortcrust pastry
1 oz. (25g) butter or margarine
4 oz. (100g) Cheddar cheese, grated 1 medium sized onion
1/3 pint (200 ml) milk
salt & pepper to taste 1 tsp (5ml) French mustard
2 eggs
1 large tomato

Roll out the pastry and use to line a 7.5” (19 cm) flan tin. Chop the onion. Melt the butter and sauté the onion until transparent. Allow to cool slightly, then sprinkle over the base of the flan. Sprinkle cheese on top. Beat the eggs, milk, mustard, salt & pepper together and pour into the flan case. Arrange the sliced tomato around the outside edge and bake in the oven at 180C (350F/Mark 4) for 40-45 minutes, until set and golden.