Preheat the oven to 425°F. Add the potatoes to a half sheet pan. Drizzle generously with olive oil and season with salt and pepper. Toss to coat. Spread the potatoes into one even layer, cut-side down. Roast for 15 minutes.

Arrange the slices of spring onion (the red/white bulb section only) over the potatoes. Continue to roast for 25 more minutes, or until the potatoes are golden brown and tender. Let cool on the sheet pan for 10 minutes before tossing the salad.

Add the arugula, radishes, and the remaining green spring onion stems to a large salad bowl.

For the pesto, combine the arugula, basil, chives, garlic, pine nuts, pecorino romano cheese, lemon zest, lemon juice, and honey in a food processor. Pulse until finely chopped. Transfer this mixture to a bowl. Stir in the olive oil. Season with salt and black pepper to taste.

Add the potatoes and onions to the salad bowl. Add enough pesto to your liking, tossing to combine, and reserve any extra for another use.

Serve the potato salad warm or at room temperature, with more black pepper over top.