Place the beets and water in an 8 by 8-inch oven-safe dish and cover with foil. Roast until a paring knife inserted in the center slides out, about 45 minutes.
Cool the beets completely, and then rub off the skin with your fingers (if the beets are fully cooked, the skins should slide right off; to make it even easier, you can do this under running water or in a bowl of cool water).
Dry and then thinly slice the beets.
REDUCTION
Add the vinegar and maple syrup to a small saucepan over medium high heat; bring to a boil, and then turn the heat down to medium low and simmer until reduced by about ⅓. Cool completely (the syrup will thicken more as it cools).
SALAD
To serve, arrange the salad greens on a large platter. Top with the sliced beets, walnuts, and blue cheese. Drizzle on the reduction to taste and serve immediately.