Add cauliflower florets, onion, leeks, garlic and stock in a large stock pot. Bring to boil, then reduce to medium and cover. Cook 10-15 minutes until the vegetables are tender. Strain out the stock, but reserve it for later use. Using an immersion blender, blend cooked veggies with salt and fat. Slowly add reserved stock to get desired consistency.
Use the white and light green part of the leek. Cut the tip off and then cut leeks in chunks. Rinse the leek layers thoroughly before cooking. Leeks are usually full of dirt and sand.