Mix until all the ingredients are incorporated completely.
If the dough is too dry add water by the teaspoon until it is workable but not soggy or sticky.
Knead lightly until the dough is smooth (about 1 minute).
Wrap in plastic film and let it rest at room temperature for at least 10 minutes.
For the filling.
Heat the oil over low heat in a large pan with cover.
Add the onions and stir until they are coated with oil. Cover and simmer over for 5 minutes, stirring often.
Increase heat to medium and add the Holland House Sherry, simmer until the sherry has mostly evaporated. Cover and simmer over low heat for another 15 minutes, or until the onions have turned caramel brown and are very tender.
Season with the garlic salt, pepper and nutmeg.
Remove from the heat. Mix in the flour and cool to room temperature.
Roll out the dough with a rolling pin until you have a circle big enough to cover the bottom and sides of a 9" [23 cm] pie pan.
Cover the pie pan with the dough and tidy up the sides.
Heat oven to 350 ºF [175 ºC].
Bake the pie crust for 15 minutes.
Remove from the oven.
Fill the pie crust with the onion, top with the cheeses.
Return to the oven and bake until the cheese is bubbly and golden brown.
If you don't have garlic salt mix ½ tablespoon of regular with ½ tablespoon of garlic powder.For a vegan or vegetarian version, use your favorite cheese substitute (I've found some pretty good vegan ones), or skip the cheese entirely. The onion is sublime on itself. Use your preferred milk substitute for the crust. I have done it with almond milk too.Whole wheat dough is very fragile, and not at all elastic like regular dough, I've found that the best way to spread it is doing it between two plastic film pieces. There's going to be some patching too. Don't worry too much, that's why I called it "rustic".