I tend to use soft, but why not. Yes you can freeze it, use a pack of frozen puff pastry. Roll and fill a large square flan dish of some kind, cover the base with sliced mushrooms, fry two or three onions and maybe a garlic love very slowly adding a little sugar and vinegar, when they are just starting to catch remove them and dpread over the mushrooms. Add the tomatoes so that they are touching all over the layer, ditto the cheese. Beat an egg, brush it over the pastry that shows and drizzle the remainder over the tart. Add lots of black pepper and herbs. Bake until the pastry and cheese are beginning to brown.