To roast the beets, preheat oven to 400 degrees F. Cut the top green tops off of the beets, if still attached. Scrub the beets clean. Wrap each beetroot in a piece of aluminum foil.
Arrange the beets on a baking sheet. Roast for 45-60 minutes, until fork tender. The cook time will vary a bit based on the size of the beets.
Remove from oven, and allow the beets to cool. Use a paper towel or clean dishcloth to rub the skins off of the beets. If the beets are fully cooked, the skin should peel away easily. Chop the beets into bite sized pieces for your salad.
Alternatively, you can use pickled beets. See the notes section for this trick.
Combine the vinegar, lemon juice, and maple syrup in a medium bowl. Whisk together. Add salt and pepper, to taste.
Add the beets to the bowl with the dressing, and stir to coat them well. Let stand for at least 30 minutes, stirring occasionally.
Prepare the Salad
Assemble the salads by tossing together the mixed greens, marinated beets, and walnuts.
Drizzle with remaining marinade for additional dressing, if desired.
Shortcut: Use a 12-ounce jar pickled beets — make sure it's not too high in added sugars, and NOT high-fructose corn syrup! Drain off a scant ½ cup of the liquid to combine with the olive oil to use as the dressing.