Heat oven to 450°F. Coat large rimmed baking sheet with 1 tablespoon oil. Arrange radishes cut sides down on sheet and drizzle with remaining oil. Roast 12 minutes.
Meanwhile, in large bowl, combine butter and miso.
Season salmon with ¼ teaspoon pepper, then sprinkle with sesame seeds, pressing to adhere.
Nestle fish among radishes, reduce oven temp to 425°F, and roast until salmon is opaque throughout and radishes are deep golden brown, 10 to 12 minutes.
Transfer salmon plates and transfer radishes to bowl with miso butter (leaving behind any oil). Toss to coat, then fold in 3 cups radish leaves until beginning to wilt. Serve with salmon.
