Make the dressing: Zest the orange and measure out 1 teaspoon. Add that to a small bowl or Mason jar. Add 1/4 cup fresh orange juice (about half a juicy orange), 2 tablespoons olive oil, 1 tablespoon rice vinegar or white wine vinegar, 1 teaspoon Dijon mustard, 1 teaspoon maple syrup, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper. Whisk or shake (if it's in a Mason jar) until combine.
Heat oven to 425°F and place beets in an oven-safe dish. Drizzle 1 tablespoon oil over beets and season generously with salt and pepper. Toss to coat beets in oil. Cover with foil and roast until a sharp knife is easily inserted in the beets, about 45 minutes to an hour. Carefully remove foil and let cool. When cool enough to handle peel beets by simply rubbing off the skin with your fingers or scraping off with a pairing knife. Cut off ends, cut into quarters, then slice in 1/2-inch thick wedges.
Place beets, 3 cup torn romaine, 1/4 of a red onion, thinly sliced , and 1/4 cup toasted walnuts in a large salad bowl.
Drizzle the dressing over the top and toss until well combined. Taste and add more salt or pepper as needed. Serve immediately.