Bring a pot of water to a boil for cooking the beans.
Mix soy sauce, brown sugar, wasabi, and garlic in an 8×8 inch pan. Add tuna and turn it several times to coat it with the marinade. Let the tuna rest for 15 minutes.
Add oil to a skillet and bring to medium hight heat.
Boil green beans for 3 minutes then drain and set aside.
Remove tuna from marinade. Reserve the soy sauce mixture for later.
Carefully add tuna to the skillet, and sear to your desired level. One minute per side will be rare. Three minutes per side will be well done and opaque.
Remove tuna to a plate.
Add marinade and green beans to the pan. Toss for a minute until coated.