Heat oil in a large frypan or saucepan. Cook onions, mushrooms and garlic in oil for 3 to 5 minutes, stirring frequently. Stir in carrots, potatoes and celery. Stir in cauliflower, green beans and vegetable stock. Bring to a boil then turn heat down to a simmer. Cook until vegetables are barely tender, about 5 minutes. Season with salt and pepper.
In a small bowl, mix the cornflour, soy sauce and 1/4 cup (65ml) water until cornflour is completely dissolved. Stir into vegetables and cook until sauce thickens, about 3 minutes.
Roll out 1/2 of the dough to line an 20x30cm baking dish. Pour the filling into the pastry lined dish. Roll out remaining dough, arrange over the filling and seal and flute the edges.
Bake in preheated oven for 30 minutes, or until the crust is brown.