Roast the Beetroot: Preheat the oven to 200°C (400°F). Wrap each beetroot tightly in foil and place them on a baking tray. Roast for 45–60 minutes or until a knife easily pierces through the beetroot. Let them cool slightly, then peel off the skins (they should slip off easily). Cut into wedges.
Toast the Walnuts: While the beetroot is roasting, spread the walnuts on a separate baking tray and toast in the oven for 5–7 minutes, or until golden and fragrant. Let cool.
Make the Dressing: In a small jar or bowl, whisk together olive oil, balsamic vinegar, honey, Dijon mustard, salt, and pepper until well combined.
Assemble the Salad: In a large serving bowl, layer the baby spinach, roasted beetroot, toasted walnuts, and red onion (if using). Crumble the goat cheese over the top.
Dress and Serve: Drizzle the dressing over the salad just before serving. Toss gently to combine, or serve with the dressing on the side.