In a small saucepan, combine the soy sauce, mirin, shallot, jalapeño, orange juice, lemon zest and juice, sugar and coriander. Simmer over moderate heat until reduced by half, about 8 minutes. Strain the sauce.
In a large nonstick skillet, heat the oil until shimmering. Season the tuna with salt and pepper. Cook over high heat for 1 minute per side. Transfer the tuna to a platter and let rest for 1 minute; thickly slice and serve with the sauce.