Core tomatoes. Chop 1 tomato and slice the other.
Stem, seed, and coarsely chop bell pepper.
Peel kohlrabi and cut into 1/2-inch chunks.
Trim ends from carrots, zucchini, and beans. Cut carrots in half lengthwise, then into 2-inch pieces. Quarter zucchini lengthwise, then cut into 2-inch pieces. Cut beans into 2-inch pieces. Chop onion.
In a 6- to 8-quart pan over medium heat, stir oil, onion, and garlic occasionally until onion is limp, about 4 minutes.
Stir in the chopped tomato, bell pepper, kohlrabi, carrots, cayenne, and pepper. Cover and simmer, stirring occasionally, until kohlrabi is barely tender when pierced, about 10 minutes.
Stir in rice and broth and bring mixture to a boil over high heat. Cover, reduce heat, and simmer until rice is almost tender to bite, about 20 minutes. Add zucchini and green beans; simmer, covered, just until beans are tender when pierced, 7 to 10 minutes longer.
Spoon onto a platter and top with sliced tomato. Add salt to taste.