Step 1

Preheat oven to 425°. Grind bonito flakes in a spice mill or blender until finely ground (you should have about 2 Tbsp.). Bring ground bonito, sugar, soy sauce, mirin, and togarashi to a boil in a medium saucepan. Reduce heat to medium and simmer, stirring often, until reduced by a third, 12–15 minutes. Pour dressing into a small bowl; cover and chill until cold, at least 30 minutes and up to 3 days. Stir in lemon zest and juice.

Step 2

Meanwhile, separate tops from turnips. Trim stems from leaves; discard. Coarsely chop leaves. Cut turnips into 1" wedges. Toss turnips and leaves with oil on a rimmed baking sheet; season lightly with salt (the dressing is salty). Roast, tossing halfway through, until leaves are wilted and turnips have taken on a touch of color, 12–15 minutes.

Step 3

Transfer turnips and leaves to a platter; spoon some of the dressing over (you’ll have extra). Sprinkle with sesame seeds.