Savoyard Leek and Sausage Flan

• pastry for 8- or 9-inch pie shell
• 1 medium-sized onion
• 1

Chop onion, ham and mushrooms finely. Place this mixture in a saucepan with salt, freshly ground black pepper and 2 tablespoons butter. Sauté gently until the onion is transparent. Set aside. Combine flour and 2 tablespo