1. Preheat oven to 375°.
2. Wrap beets in foil. Bake at 375° for 1 1/2 hours or until tender. Cool; peel and thinly slice.
3. Place farro and 3 cups water in a medium saucepan; bring to a boil. Reduce heat, and simmer 25 minutes or until farro is tender. Drain and cool. Stir in 1/2 teaspoon salt.
4. Arrange 1 1/2 cups watercress on a serving platter; top with half of farro, 1/4 cup onion, and half of sliced beets. Repeat layers with remaining 1 1/2 cups watercress, remaining farro, remaining 1/4 cup onion, and remaining beets. Sprinkle top with cheese.
5. Combine remaining 1/4 teaspoon salt, vinegar, and remaining ingredients; stir well with a whisk. Drizzle vinegar mixture evenly over salad.