Removed stems from mushrooms, place in bowl full of water & microwave on high for 3-5 minutes, then drain & set aside.
Clean leeks & chop roughly. Heat oil in skillet and cook leeks, thyme for 5 minutes. Add mushrooms. Cook until leeks begin to brown.
In baking dish, combine leeks, mushrooms, mustard and mayonnaise. Season with salt & pepper. Sprinkle cheese on top (and drizzle with olive oil - optional) Bake 350 for 20 minutes. Can also be baked in pie shell.