We heat a medium-sized pan with the oil over medium heat. When hot, the chopped spring onions are added. We stir for a few minutes, lowering the heat as necessary. We want the onions to become tender but not browned. Next, the vinegar is added directly to the hot pan (we are careful to stand back from the fumes!).
The hot vinegary onion mixture is tossed with the arugula. The salad is drizzled with the walnut oil and sprinkled with salt and freshly ground black pepper.