For crust combine oats and bran, cut in butter with a pastry cutter or 2 knives to coarse meal stage.
Add water, stir just until moist.
Press into a ball on wax paper and cover with more wax paper.
Roll dough, still covered into 10 inch circle.
Remove 1 sheet of wax paper and fit into a 9 inch greased pie plate. Remove other wax paper.
Bake for 7 minutes. Cool on wire rack.
For filling, put skillet on medium high heat and add a bit of oil. Add leak and saute for 2 minutes. Add mushrooms and saute for 5 minutes. Remove from heat and put in bowl.
Combine milk and everything else except the Gruyere in a blender and process until smooth. Add to mushroom mixture and stir well.
Pour into crust and sprinkle with Gruyere.
Bake 35 minutes or until a knife inserted near center comes out clean.