Place carcasses in a large pot. Add onion, carrot, celery, garlic and water. Bring to a boil, skimming foam. Reduce heat to low and add herbs, peppercorns and salt; simmer gently, uncovered, for 2 hours. Strain through a fine sieve into a large bowl. (Discard solids.) Let cool slightly, then refrigerate until cold. Skim off fat.