Look for the freshest sushi-grade tuna available to make this raw dish.Plus: More Seafood Recipes and Tips
In a saucepan, combine the orange and lemon juices and boil over high heat until reduced to 1/3 cup, 7 minutes. Transfer to a bowl; let cool. Stir in the vinegar, soy, jalapeño and ginger. Whisk in the olive oil; season with salt and pepper.
In a small skillet, toast the sesame seeds over moderately high heat, shaking the pan until golden, 2 to 3 minutes.
Cut the tuna into 4 equal pieces. Place 1 piece of tuna between 2 sheets of plastic and pound to a 1/8-inch thickness. Remove the top sheet of plastic and invert the tuna onto a plate. Repeat with the remaining tuna.
Stir the dressing and spoon it over the tuna. Garnish with the radishes and sesame seeds and serve.