Place the carrots, onion, cauliflower, and cashews in a large pot. Fill it with enough water to cover the vegetables plus about two inches.
Bring the water to a boil over medium-high heat and then reduce the heat and allow to simmer for about 15 minutes, or until the vegetables are fork tender.
Drain the vegetables and save the water. Allow the vegetables to cool slightly and then place them in a high-speed blender or food processor along with the nutritional yeast, garlic, cornstarch, lemon juice, mustard, and salt. Add 2 cups of the reserved cooking water and blend until smooth and creamy. Add a little more water if the mixture is too thick. Keep in mind that it will thicken when it’s heated.
Pour the sauce in a medium-sized sauce pan and place it over medium-high heat. Bring the sauce to a boil, reduce heat and allow to thicken slightly.
Remove from heat. Mix cheese sauce in with pasta for macaroni and cheese, drizzle it over a baked potato, or add it to any dish that could use a little cheese goodness.