Mince garlic and finely chop shallot and onion. Cut bell peppers into julienne strips. Cut tomato into 1/4-inch dice. Scrub clams and mussels and remove beards from mussels. Shell and devein shrimp.
In a 4-quart shallow heavy kettle with a tight-fitting lid cook garlic, shallot, onion, and bell peppers in oil, uncovered, over moderate heat, stirring, 5 minutes, or until peppers are softened. Add vermouth, wine, and saffron and boil, uncovered, until liquid is reduced to about 1/3 cup. Add stock or clam juice and cream and bring to a boil. Immediately add clams and simmer until they just begin to open, about 3 minutes. Stir in mussels, shrimp, tomato, and sea salt and pepper to taste and stir until combined well. Simmer cataplana, covered, stirring occasionally, 5 minutes and discard any unopened clams or mussels. Transfer seafood with a slotted spoon to a large bowl and boil cooking liquid, uncovered, until reduced by about half, about 5 minutes. Return seafood to cooking liquid and stir in crayfish and sorrel. Heat cataplana over moderate heat until just heated through.
