Step 1: Roast the beets: Preheat the oven to 400°F. Scrub the beets well and trim off the leafy tops, leaving the skin on. Place the beets on a piece of aluminum foil inside a rimmed baking sheet or in a baking dish and drizzle lightly with olive oil and sprinkle with kosher salt. Fold the foil into a tight packet around the beets and roast until a knife slides easily into the center. The exact cooking time depends on the size of your beets. Smaller beets may take around 40 minutes, while larger ones can take an hour or more. Don't rely on the clock alone. The knife test is what matters.
Step 2: Cool, peel, slice & chop: Let the roasted beets cool enough to handle. The skins should rub off fairly easily with your fingers or a paper towel. If a stubborn spot doesn't want to cooperate, use a knife. Cut the beets into wedges or bite-sized pieces. Thinly slice shallots or red onion and coarsely chop nuts for garnish. Tip: If you're using red and golden beets, keep them separate while cutting. Red beets will happily turn everything around them pink.
Step 3: Segment the citrus: Cut the top and bottom from each citrus fruit. Set the fruit upright and use a sharp knife to cut away the peel and white pith. Then cut between the membranes to remove the individual segments. Work over a bowl to catch the juice. Save some of that juice for the vinaigrette. This extra step is worth it. Citrus segments are much more pleasant in a composed salad than large pieces of peel and pith, and you get fresh juice for the dressing at the same time.
Step 4: Make the vinaigrette: Whisk together the apple cider vinegar, citrus juice, Dijon mustard, honey or maple syrup, garlic powder, salt, and black pepper. Whisk in the olive oil until the dressing is combined, or shake vigorously in a jar. Taste it before dressing the salad. It should taste bright and slightly tangy with enough sweetness to balance the vinegar.
Step 5: Assemble the salad: Arrange the salad greens on the bottom, then layer in roasted beets and citrus segments on a large platter or shallow serving bowl. Scatter the thinly sliced red onion, crumbled feta, and fresh herbs over the top. Drizzle with the apple cider vinegar vinaigrette just before serving. If you're adding toasted nuts or seeds, sprinkle them over the salad at the very end so they stay crunchy. Finally, sprinkle on some orange zest if you'd like a final picture-perfect touch.