Preparation 1. Melt butter in large pot and simmer onions until very soft and golden (about 25 minutes), stirring often.2. Add the chicken stock and bring to a boil3. Trim tips from asparagus and reserve. Cut about 1 inch from the butt ends of the asparagus spears; don't try to remove all the tough parts; just the very woody ends. Chop spears into 1/2 inch pieces and drop into boiling chicken stock. Cover, reduce heat, and simmer for 45 minutes, or until asparagus is very soft.4. Force the soup, broth and all, through the medium disk of a food mill. Return puree to the pot, add the reserved asparagus tips, and simmer until they are tender but still firm, 5 to 10 minutes. If serving the soup hot, season with salt and pepper and serve.5. If serving the soup cold, remove from heat, cool, stir in the cream or buttermilk, and refrigerate covered. Season to taste with salt and pepper. Serve very cold.