1. Place farro in a large saucepan; cover with water 2 inches above farro. Bring to a boil. Cover, reduce heat, and simmer 28 minutes. Add beans to pan; cook 2 minutes or until crisp-tender. Drain and rinse with cold water; drain.
2. Combine shallots and next 5 ingredients (through garlic) in a medium bowl, stirring well with a whisk. Add farro mixture, corn, and oregano; toss gently to coat. Stir in 1/4 teaspoon salt and 1/4 teaspoon pepper.
3. Arrange 3 tomato slices on each of 8 plates; sprinkle evenly with remaining 1/2 teaspoon salt and remaining 1/4 teaspoon pepper. Top each serving with about 1/4 cup farro mixture.