To make the tart shell, mix the flour, cornmeal, salt and black pepper in a bowl.
Cut the coconut oil into the all purpose flour with a fork or a pastry cutter until you have small pieces of the fat dispersed throughout the flour.
Drizzle in ice-cold water and mix with a fork until the dough comes together in a ball. Add the water slowly because you don't want a wet dough-- it should be moist enough to just hold together.
Press the dough into a flat disc and wrap tightly in cling wrap. Refrigerate for at least 30 minutes.
In the meantime, make the filling.
Heat the oil in a large saucepan and add the onions, sugar and a little salt. Cook, stirring frequently, until the onions start to caramelize and become golden brown. This took me about 20 minutes over medium heat.
When the onions are browned, stir in the thyme, then add the cognac to deglaze the pan and scrape up all the tasty brown bits stuck at the bottom. If you'd rather not use cognac, use vegetable stock or plain old water.
Turn off the heat and set the onions aside to cool.
In a blender, crumble in the tofu then add soymilk, nutritional yeast if you're using it, mustard and salt to taste. Blend until you have a very smooth mixture.
Scrape the tofu mixture into the pan with the onions and mix well.
To assemble the tart:
Preheat the oven to 400 degrees Fahrenheit.
Remove the tart dough from the refrigerator and roll it out on a floured surface to a diameter two inches larger than the base of the tart pan (mine is 9 inches wide). Fold the dough in half and then transfer to a tart pan and open it so it lies evenly across the pan. If the dough tears, don't worry. Patch it up once you have transferred it to the pan. No one will be able to tell.
Run the rolling pin across the edges of the tart pan and remove any excess dough hanging over the sides.
With a fork, prick the bottom of the tart shell in several places. Then cover with aluminum foil and fill it to the brim with rice or beans or pie weights. I keep some rice in a box and reuse it every time I need to blind bake a pie or tart crust.
Bake the filled tart crust for 10 minutes, then carefully remove the foil with the rice or beans in it, and continue to bake the crust for another 10 minutes until it is quite dry.
Remove the crust from the oven and increase the heat to 425 degrees. Brush the bottom of the crust with some extra virgin olive oil. Now scrape in the filling and smooth it.
Return the tart pan to the oven and bake for 40 minutes until the top of the tart is golden-brown.
Remove the tart to a rack and cool 10 minutes. Then unmold it -- I do that by placing the tart on top of a mason jar and letting the rim slide off. Slide the tart off the base and onto the rack and continue cooling it for another 10 minutes.
Serve warm or at room temperature.