Preheat oven to 400°F (200°C). Wrap each beet individually in aluminum foil and place on a baking sheet.
Roast beets in the preheated oven for 45–60 minutes, or until tender when pierced with a fork. Let cool, then peel and slice into wedges or cubes.
In a small bowl, whisk together balsamic vinegar and Dijon mustard. Slowly drizzle in olive oil while whisking continuously to emulsify. Season with salt and pepper.
In a large salad bowl, combine mixed greens, roasted beets, goat cheese, and toasted walnuts.
Drizzle the balsamic vinaigrette over the salad and toss gently to combine. Serve immediately.
Notes
For best flavor, toast the walnuts in a dry skillet over medium heat for 3–5 minutes until fragrant.