Preheat oven to 375°.
To prepare crust, combine oats and oat bran; cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Add water; stir just until moist. Press mixture gently into a ball on wax paper, and cover with additional wax paper. Roll dough, still covered, into a 10-inch circle.
Remove 1 sheet of wax paper, and fit dough into a 9-inch pie plate coated with cooking spray. Remove top sheet of wax paper. Bake at 375° for 7 minutes. Cool on a wire rack.
To prepare the filling, place a medium nonstick skillet coated with cooking spray over medium-high heat until hot. Add leek; sauté for 2 minutes. Add mushrooms; saute for 5 minutes. Remove from heat; spoon into a bowl.
Combine milk and next 8 ingredients (milk through spinach) in a blender, and process until smooth. Add to mushroom mixture, and stir well. Pour into prepared crust, and sprinkle with Gruyêre cheese. Bake at 375° for 35 minutes or until a knife inserted near the center comes out clean. Let stand 5 minutes.