Cook 6 ounces of potato. Poke it several times with a fork and microwave for 3-4 minutes or steam or boil. (If you have to cook more than 6 ounces, measure it afterwards and only use one heaping half cup.)
Allow the potato to cool until you can comfortably peel it. Mash it well in a bowl with 1 tablespoon milk and nutritional yeast. Put it in the refrigerator to cool while you mix the dry ingredients.
Preheat oven to 425 and line a baking sheet with parchment paper or silicone mat. Combine the flours, baking powder, soda, and salt. When the potato is no longer warm, add it to the flour and "cut" it in using a fork or pastry blender until the mixture is crumbly.
Combine the non-dairy milk and lemon juice and add it, stirring until a soft dough forms. If all flour is not moistened, add additional milk a teaspoon at at time until dough can form a ball.
Turn out the dough onto a well-floured surface and knead it 4 times. Flatten it down and sprinkle the top lightly with flour. Roll it out about 3/4 inch thick. Using a 1 1/2-2-inch wide biscuit cutter or glass dipped in flour, press straight down without twisting to cut into 12-15 biscuits.
Place the biscuits, sides lightly touching, on the prepared sheet. Bake until tops are lightly browned, 9-11 minutes. Serve hot.
How much extra milk you need will depend on how moist your potato is. If you are boiling or steaming the potato, you probably won't need much extra milk.For "cheesier" biscuits, use 3 tablespoons of nutritional yeast and add one teaspoon of tahini to the potato.