In a 7-qt/6.5-L slow cooker, combine the onions, leeks, carrots, celery, fennel, garlic, tomato paste, peppercorns, thyme, and bay leaves. Cover with 12 cups [2.8 L] of cold water. Cover the cooker and cook on low heat for 8 to 9 hours.
Strain the broth through a fine-mesh sieve into a large bowl. Add the soy sauce. Refrigerate, uncovered, until the broth has c