Roast the beets: Heat your oven to 400°F (200°C).
Rub the beets with 1 tablespoon olive oil and a pinch of salt, then wrap them tightly in foil (or place in a covered baking dish). Roast for 45–60 minutes, until a knife slips in easily.
Cool and peel: Let the beets cool until you can handle them. Use a paper towel to rub off the skins.
Trim the ends, then cut into wedges or 1/2-inch cubes.
Make the vinaigrette: In a small bowl or jar, whisk the olive oil, vinegar, citrus juice, Dijon, honey, garlic, salt, and pepper until emulsified. Taste and adjust acidity, sweetness, and salt.
Prep the add-ins: Thinly slice the red onion. Toast the nuts in a dry skillet over medium heat for 3–4 minutes, stirring, until fragrant.
Let cool, then chop.
Toss the greens: In a large bowl, toss the arugula with about half the vinaigrette and a pinch of salt. You want the leaves lightly coated, not soggy.
Add beets and onions: Gently fold in the warm (not hot) beets and sliced onion. Warm beets absorb dressing nicely but won’t wilt the greens if they’ve cooled slightly.
Finish and serve: Scatter the goat cheese and nuts over the top.
Drizzle on more vinaigrette to taste and grind over black pepper. Serve right away.