Preheat your oven to 400°F (200°C). Wrap each beet individually in aluminum foil and place them on a baking sheet.
Roast the beets for 50-60 minutes, or until they're tender when pierced with a fork. Let them cool enough to handle.
While the beets are cooling, whisk together the olive oil, balsamic vinegar, Dijon mustard, and honey in a small bowl. Season with salt and pepper to taste.
Once the beets are cool, peel them by gently rubbing the skin off with paper towels. Cut them into wedges or 1/2-inch cubes.
In a large bowl, arrange the mixed greens. Top with the roasted beets, crumbled goat cheese, toasted walnuts, and sliced red onion.
Drizzle with the prepared dressing just before serving.