Make the quiche crust:
In a bowl, mix together the potato, flour, salt, pepper and thyme. Add the oil and mix with your fingers until the dough looks slightly crumbly.
Drizzle in cold water while mixing with a fork, until the dough just comes together. You can also do this in a food processor, but be careful not to over-process.
Turn out the dough on plastic wrap and tightly wrap it, shaping it into a disc as you wrap.
Place in the refrigerator for at least half an hour.
Make the filling:
Heat the oil, add the onions and garlic with a generous pinch each of salt and black pepper, and sugar.
Saute, stirring frequently, until the onions caramelize and turn golden brown. Set aside to cool.
In a food processor, place the tofu and nondairy milk and process until very smooth.
Add the Sriracha, 8 sundried tomatoes, and lemon juice. Add the cooled onions and garlic. Process until the ingredients break down into small pieces but are still discernible. This will give your filling a great texture and flavor. Add salt to taste.
Assemble the quiche:
Preheat the oven to 350 degrees Fahrenheit.
Flour a flat surface and roll out the quiche crust into a diameter a couple of inches larger than a 9-inch tart pan.
Fold the dough in half and then half again and transfer to the tart pan. Open the dough, push it with your fingers into the sides of the tart pan so it fits snugly, then remove any overhanging dough by rolling over the edges with your rolling pin and pulling out the excess dough.
Pour the filling into the quiche crust and smooth the top. Sprinkle on the remaining sundried tomatoes and a few sprigs of thyme for a decorative look
Bake in the preheated oven for 40 minutes or until the top feels firm.
Remove to a rack and serve warm or cold or at room temperature. It's delicious every way.