Prepare the dough by first adding the vegan butter into the flour and salt until the mixture resembles coarse crumbs. Add the water and knead for about a minute, just until everything comes together. Try not to overwork it. Let it chill in the fridge while you prepare the filling.
Slice the onions.
Melt a little vegan butter in a large frying pan and cook the onions over medium heat for at least 10 minutes, until they're nicely browned and caramelised.
While the onions are cooking, roll or press the dough into a quiche or cake pan. Prick the base all over with a fork and prebake it at 180°C (350°F) for 10 minutes.
While the crust is prebaking, add the spinach, thyme, salt, and pepper to the onions. Cook for another few minutes until the spinach has wilted and everything is soft and fragrant.
Don't forget to take the quiche crust out of the oven. Add the tofu, soy milk, water, lemon juice, nutritional yeast, cashews, chickpea flour, starch, turmeric, salt, and kala namak (if using) to a blender. Blend until completely smooth. If the mixture seems too thick, add a splash more water.
Stir the onion and spinach mixture into the blended filling, then pour everything into the prebaked quiche crust. Spread it out evenly and arrange the tomato slices on top.
Bake for 30 minutes at 180°C (350°F). Once baked, let the quiche cool for at least 30 minutes before slicing and serving. The longer it rests, the better the texture will be. Enjoy!