Cook soba noodles according to package directions, drain, rinse well under cold water. Allow to drain and then toss with soba sauce. Set aside.
In a skillet over medium heat, cook the onions until slightly softened. Add vegetables, jalapeño, ginger and garlic, and stir-fry until vegetables are tender.
Season tuna with salt, pepper and Togarashi. Sear tuna on grill or skillet set on high heat. Do not overcook (tuna should be pink inside), about 30 seconds per side.
Place soba in center of plate and top with vegetables. Slice tuna and serve alongside.