Break apart the tofu block into 4 pieces and add into food processor. Process the tofu until smooth and creamy. If it doesn't get creamy, add a tiny splash of almond milk to help it along.
In a skillet, add oil and saute the leek (or onioand 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, but not to worry.
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.