Preheat oven to 400 F and wrap each beet in a piece of tin foil.

Roast the beets for 1 hour or until tender and let cool to room temperature. Once cooled, peel beets, and cut into 1x1 inch pieces.

Mix 1 cup distilled vinegar with salt and let the beets soak in the vinegar and salt overnight.

The next day, drain the vinegar, and chop watercress. Mix in a large mixing bowl, and serve with a drizzle of olive oil.