Preheat oven to 450ºF. Peel beets; cut into batons about 3 inches long and ¼ inch thick. Set aside beet greens for serving (if purchased in tact). Toss beets and onion with olive oil; place on a cookie sheet. Roast for 20-25 minutes, stirring occasionally, or until onion is brown tipped. Cool to room temperature.
Stir honey with mustard in a medium bowl until combined; slowly whisk in lemon juice and salt. Add beets and onions, including any liquid; gently toss to coat.
If making ahead, cover and refrigerate salad for up to 2 days; bring to room temperature before serving. Serve on a bed of tender beet greens or baby spinach generously scattered with pumpkin seeds.