While your quiche is baking, make the sauce.
To your blender, add the oats, sunflower seeds, nutritional yeast, tapioca flour, onion powder, garlic powder, salt, mustard powder, white pepper, and nutmeg. Blend for about 20 seconds, until the ingredients come together into a fine powder.
Leave the powder in the blender and add the water, hemp seeds, non-dairy milk, miso, lemon juice, apple cider vinegar, tahini, and dijon mustard. Blend again until completely blended and smooth.
Transfer the mixture to a saucepan. Cook over medium heat, stirring often, for 10–12 minutes, or until the mixture thickens and achieves a thick, glossy, gooey cheese-like texture. Stir the sauce more consistently as it begins to thicken. If it doesn’t get thick at first, wait for it, it will. It may have lumps in the beginning, but it will smooth out as it thickens.