Combine first 9 ingredients in a shallow dish or a heavy-duty zip-top plastic bag; add black beans and green beans. Cover or seal, and let mixture stand 30 minutes.
Cut a 1/4-inch slice from top of each tomato; scoop out and discard pulp, leaving tomato shells intact. Invert tomato shells, and drain on paper towels.
Place each shell on a salad plate. Spoon 1/4 cup bean mixture into each tomato shell; spoon remaining mixture evenly on plates around shells. Garnish, if desired.