Roast the beets: Heat the oven to 400°F. Quarter the beets (halve the small ones), toss with the olive oil, 1/2 tsp of the salt, and the pepper, and arrange cut-side down on a rimmed baking sheet. Roast 35–40 minutes until a knife slides in easily and the cut faces are deeply caramelised. Cool 10 minutes, then rub off the skins.
Toast the pistachios: In a dry skillet over medium heat, toast the pistachios 2–3 minutes, shaking often, until fragrant and lightly browned. Cool completely, then roughly chop.
Whip the goat cheese: In a food processor (or with an electric hand mixer), blend the goat cheese, cream cheese, honey, lemon zest, and a pinch of salt until completely smooth and mousse-like, 1–2 minutes. Refrigerate until serving.
Whisk the dressing: Whisk the balsamic vinegar, honey, Dijon, remaining 1/4 tsp salt, and pepper. Drizzle in the extra-virgin olive oil while whisking until glossy and emulsified. Adjust honey or vinegar to taste.
Prep the oranges: Cut the peel and white pith from the oranges, then slice into wheels or supremes over a bowl to catch the juice. Stir a spoonful of that juice into the dressing for brightness.
Assemble the salad: Spread the arugula over a large platter. Arrange the roasted beets and orange slices on top, add generous dollops of whipped goat cheese, and drizzle with the dressing. Shower over the pistachios, finish with a thin thread of honey, and serve right away.