Use a potato peeler to peel the skin in strips from two well washed, Seville oranges. Cut into fine julienne and blanch in a large quantity of boiling water for 8 minutes. Refresh under cold running water and drain. Squeeze the juice from all 4 oranges and set aside.
Heat 3 tablespoons of butter in a heavy saucepan until it froths, add the flour and cook until the butter turns a light g