Preheat oven to 350F and lightly grease a round 10-inch tart pan. Alternatively, you can use a 9-inch glass pie dish if desired.
Wrap rinsed tofu in a few tea towels. Place a few books on top of it to lightly press out the water while you prepare the crust.
To prepare the crust, whisk together flax and water mixture in a small bowl and set aside so it can gel up.
In a large bowl, stir together the almond meal, oat flour (or buckwheat flour), parsley, oregano, and salt.
Add in the flax mixture and oil. Stir until mostly combined, adding the remaining water until the dough is sticky (about the consistency of cookie dough). The dough should stick together when you press it between your fingers.
Crumble the dough evenly over the base of the tart pan (or pie dish). Starting from the centre of the pan, press the mixture evenly into the pan, working your way outward and up the sides of the pan. Poke a few fork holes in the dough so air can escape.
Bake the crust at 350F for 13-16 minutes, or until lightly golden and firm to touch. Set aside to cool while you finish preparing the filling. Increase oven temperature to 375F.
To prepare the filling, break apart the tofu block into 4 pieces and add into food processor. Process the tofu until smooth and creamy. If it does not get creamy, add a tiny splash of almond milk.
In a skillet, add oil and sauté the leek (or onion) and garlic over medium heat for a few minutes. Stir in the mushrooms, season with salt, and cook on medium-high heat until most of the water cooks off the mushrooms, about 10-12 minutes.
Stir in the herbs, sun-dried tomatoes, spinach, nutritional yeast, oregano, salt, pepper, and red pepper flakes until combined. Cook until the spinach is wilted.
Finally, remove from heat and stir in the processed tofu until thoroughly combined. Adjust seasoning to taste if desired. Spoon mixture into baked crust and smooth out with a spoon until even.
Bake quiche, uncovered, at 375F for 33-37 minutes, until the quiche is firm to the touch. For best results, cool the quiche for 15-20 minutes on a cooling rack before attempting to slice. The crust may crumble slightly when sliced warm.
Wrap up leftovers and refrigerate for 3-4 days. Leftover quiche can be reheated in the oven on a baking sheet for about 15-20 minutes at 350F.