Peel the sweet potato and cut it into cubes. Remove the woody end from the green asparagus. Mix the tamari with the ground ginger in a bowl. Place the tuna steak in it, coat the fish well with the marinade and let it marinate briefly.
Bring a pan of water to a boil. Cook the sweet potato and quinoa in it for twelve minutes until tender and the liquid is absorbed. Meanwhile, heat a grill pan on the stovetop and add the olive oil. Grill the green asparagus in the hot pan until tender-crisp. Remove the asparagus from the pan and set aside.
Place the marinated tuna steak in the same hot grill pan. Sear the fish for one minute per side so the outside gets a brown crust and the center remains rare. Remove the tuna from the pan immediately to prevent further cooking.
Spoon the cooked sweet potato and quinoa onto a plate. Place the grilled asparagus beside it and top with the tuna steak. Finally, sprinkle the dish with sesame seeds and freshly ground black pepper.