Heat oil in a large skillet 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 broth. 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 cornstarch, soy sauce, and 1/4 cup water until cornstarch is completely dissolved. Stir into vegetables, and cook until sauce thickens, about 3 minutes.
Roll out 1/2 of the dough to line an 11x7 inch 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.