Preheat oven to 350 degrees. In a large pot, simmer fish broth with garlic, bay leaf and salt for 15 minutes. In a 9-by-13-inch baking dish, scatter red onion and half of the thyme. Place salmon on top, skin side down, and season with salt and pepper. Scatter remaining thyme, raisins and lemon on top. Stir vinegar and sugar into broth and let simmer for 1 minute to dissolve sugar. Remove garlic and bay leaf from broth; carefully pour broth over fish. Bake for 8 to 10 minutes, until fish is just cooked through to taste. Serve hot, warm or cold, with challah for dipping into broth.