Prepare the Beets: Preheat your oven to 400°F (200°C). Trim the beet greens, wash the beets, and wrap each one in foil. Place on a baking sheet and roast for 45–60 minutes, or until tender when pierced with a fork. Let cool, then peel and cut into wedges or bite-sized cubes.
Toast the Walnuts: Spread walnuts on a baking sheet and toast in the oven at 350°F (175°C) for 8–10 minutes, stirring once, until fragrant. Let cool and chop roughly.
Make the Dressing: In a small bowl, whisk together olive oil, balsamic vinegar, Dijon mustard, honey, salt, and pepper until emulsified. Adjust seasoning to taste.
Assemble the Salad: On a large platter or individual plates, arrange the arugula or mixed greens. Top with beet pieces, crumbled goat cheese, toasted walnuts, and red onion (if using). Drizzle the dressing over the salad just before serving.
Serve: Toss gently to combine, or leave layered for a beautiful presentation. Serve immediately with crusty bread or as a side to your favorite protein.