Bitter radicchio and a rich tuna steak want each other, and a hot oven is what brings them together. The radicchio goes in alone for eight minutes, and the steaks are laid on the same hot sheet pan for the six that follow. One pan, 22 minutes, dinner for four.
Heat is what turns radicchio from harsh to sweet. Cut the head through the core into wedges so they hold together instead of falling into loose leaves, and keep each wedge sitting on a cut face rather than tumbling them around the pan. The flat side against hot metal is where the browning happens, and the browning is the point.
Whisk the honey and vinegar while the radicchio roasts and spoon it over everything at the end rather than before, so it stays sharp instead of scorching in a 425°F oven. Endive or thinly sliced fennel works in place of the radicchio, and swordfish cut to the same thickness roasts in the same time.
