To make pastry, put flour, butter and cheese in a food processor and process until mixture resembles fine breadcrumbs. Add egg yolk and water, and process until mixture comes together. Knead on a floured surface and roll out until 5mm thick. Line a rectangular 27cm x 14cm x 4cm deep silicone tray with pastry. Trim edges and put in the freezer for 30 minutes.
Preheat oven to 180°C. Heat oil in a large non-stick frying pan over a medium heat. Add leek and cook until soft but not brown. Remove from pan. Thinly slice shiitake mushrooms. Trim enoki and oyster mushrooms.
Melt butter in pan over a medium heat. Add garlic and all mushrooms except enoki and oyster. Cook, stirring occasionally, for 5-6 minutes or until beginning to colour. Stir in the leaves from 3 thyme sprigs. Season.
Put silicone tray onto an oven tray. Cover bottom of pastry case with leek, then top with gruyère. Add cooked mushrooms, then arrange enoki and oyster mushrooms in patches on top.
Whisk together eggs and cream, and pour over the top. Bake for 40-45 minutes or until set and golden. Cool slightly, then carefully remove from silicone tray. Sprinkle with remaining thyme to serve.
Note: Gourmet mushroom mixes can be made up of enoki, Swiss brown, shiitake and pink or white oyster mushrooms. You can substitute any mushrooms with extra button or Swiss brown mushrooms.