Line a 10 inch tart pan with a removable bottom with the pie crust. Trim off the excess. Using a form, prick the bottom of the crust all over. Line the crust with foil and add weights or dried beans to keep the crust from puffing up. Bake for 15 minutes or until lightly browned. Remove from the oven.
Heat the oil in a large nonstick skillet over medium-high heat. Add the leek, pepper and potato; saute 4 minutes. Spreak the leek mixture into the baked crust.
Place the milk, eggs, cheeses and mustard in a blender or food processor and process until smooth. Pour mixture over the leek mixture. Bake at 375 degrees for 40 minutes. Let stand for 10 minutes before serving.