How to Make It
Bring a medium saucepan of salted water to a boil. Add the leeks, cover and simmer over low heat until tender, about 10 minutes (3 minutes for baby leeks). Using a slotted spoon, transfer the leeks to paper towels to drain. Cut the leeks lengthwise into long, thin strands.
Add the beans to the saucepan. Cook until just tender, 4 minutes. Drain and pat dry.
Put the arugula in a large bowl. In a large skillet, heat the butter and cook over moderately high heat until richly browned, about 2 minutes. Add the leeks, green beans, vinegar, red onion and radishes and toss until warmed. Spoon the vegetables and butter dressing over the arugula, toss well and season with salt and pepper. Arrange the salad on 4 plates, garnish with the mint leaves and serve.