Make a light roux from the butter and flour. Dilute with milk. Maha from heat and add half of the feta cheese, salt, pepper and mustard. Pre-baked slices, cut each of four. Arranged on a tray, pour sauce, sprinkle with remaining feta cheese and bake. Another option: Add 4 pieces of feta cheese or 4 tomatoes, peeled and cut into pieces. Instead of bread can be used baguette.