Place carrots, potatoes, onion, garlic, and butter in a small pot and cover with the milk. Bring to a boil, then reduce heat and simmer until the vegetables are soft, about 12 minutes. Remove from heat and carefully pour into the blender. Add the chickpeas, nutritional yeast, lemon juice, mustard, turmeric, and salt. Remove the plastic knob from the blender top and partially cover with a towel, allowing the steam to escape while blending. Blend for about 1 minute, until completely smooth and glossy. Taste and add more salt or lemon juice if you like. Serve over nachos, baked potatoes, steamed veggies…!