Add the onion, carrots and sweet potato to a small pot and cover with water. Bring to a boil, cooking until fork tender, approx 10 minutes. Drain but reserve the cooking water.
Transfer the cooked vegetables to your blender along with the nutritional yeast and all the seasonings and start with 1/2 cup of the cooking water. Process until smooth adding more water 1-2 tablespoons at a time if needed. Do not add all the water at once, or you may make the sauce to thin.
Reserve the water used to cook the vegetables, but do not add all of it to your blender when making your sauce. Start with half a cup and add more if needed until you achieve your desired consistency.This recipe makes approximately 3-4 cups of cheese sauce, depending on the size of your vegetables and how much water you use to thin out your sauce.