Preheat a nonstick pan on medium-high heat, add Canola or Vegetable oil to pan.
Sprinkle outside of tuna steak with salt and pepper and cook on preheated pan, turning to a new side as each side browns. The goal is not to cook it through, but to sear the edges.
Remove heat from pan and allow to rest while assembling the tacos with sliced radishes, slaw mix, and toppings of choice.
Slice tuna and add to tacos, garnish with a squirt of lime juice and additional toppings as desired.