Pre-heat oven to 350 degrees.
In a large pan over medium heat, cook the sliced onions with the vegetable oil, balsamic vinegar, and ½ teasppon of salt. Cook for 15 to 20 minutes, stirring occasionally. The onions will brown become fragrant. Add the mushrooms and another ½ teaspoon of salt and cook for another 15 to 20 minutes, until they also become brown and fragrant.
While the vegetables are cooking you can make the crust. Oil a 10 inch pie pan or tart pan. Combine the crust ingredients in a bowl and kneed until mixed. Roll onto a lightly floured surface until it’s about 13 inches across. Gently pick up the dough and place in the pie pan. Press into all of the corners and trim away an excess. Set aside.
In a food processor, blend together the tofu, lemon juice, garlic, stock, basil, ½ teaspoon salt and nutritional yeast. Process until the mixture is thick and looks like ricotta cheese. Add more vegetable stock if the mixture is too thick. Add the spinach and pulse until it’s mixed in.
To assemble the tart, spread the tofu mixture in the bottom of the crust. Add the mushrooms and onions, pressing down slightly with a spatula.
Cover the tart with foil and cook for 30 minutes. Remove the foil and bake for another 15 minutes.
Sprinkle the chopped chives over the tart before serving. Serve hot.