Trim 2 bunches radishes, then slice as directed (see below). Place in a 9-by-13-inch baking dish and drizzle with 4 teaspoons toasted sesame oil. Season with 1/2 teaspoon salt and a few grinds of pepper and toss to coat. Roast at 425 degrees F until tender, 35 to 40 minutes. Whisk 2 tablespoons vegetable oil with 1 tablespoon mirin, 2 teaspoons each white miso paste and lemon juice, 1/4 teaspoon salt and a few grinds of pepper. Drizzle over the radishes and top with sesame seeds and sliced scallions.
Cut a thin sliver from one side of the vegetable so it can sit flat. (If you're starting with halved veggies, skip this step.)
Position the vegetable cut-side down between the handles of two wooden spoons.
Using a sharp knife, make crosswise cuts (1/8 to 1/4 inch apart), stopping when the knife hits the handles so you don't cut all the way through the vegetable.