Place the flour, butter and salt into a large clean bowl. Rub the butter into the flour with your fingertips until the mixture resembles fine breadcrumbs, working as quickly as possible to prevent the dough becoming warm.
Add the water to the mixture and using a cold knife stir until the dough binds together; add more cold water a teaspoon at a time if the mixture is too dry.
Wrap the dough in cling film / plastic wrap and chill for a minimum of 15 minutes and up to 30 minutes.
Preheat the oven to 180 C /Gas 4. Grease a tart dish.
Remove the pastry from the fridge and roll out the dough and spread evenly in a dish. Line with baking paper and place dried beans inside so as to keep the pastry from losing its shape.
Bake blind for 10 minutes. Remove from the oven.
For the filling:
Fry the onions, chilli and mushrooms in olive oil until they are brown, then add the beans. Stir for 3 minutes. Add the tinned tomatoes, chopped pumpkin and add some water, then bring to the boil.
Add the spices and black pepper, put a lid on the pot and simmer for 5 minutes. When the sauce has thickened, remove from the heat and spoon it into the prepared pastry case.
Slice the mozzarella and place on the tart, and then add the grated cheese and bake in the oven for 15 minutes or until the cheese is golden brown.
For this dish to be 100% vegetarian, check to make sure that the cheeses you are using do not contain animal rennet.