How to Make It
In a large skillet, heat 1 tablespoon of the safflower oil until shimmering. Add the sliced ginger, shallots and garlic and cook them over moderate heat until softened, about 3 minutes. Add the red grapefruit, orange and lemon juices along with the wine, light soy sauce and stock and bring to a boil over high heat. Cook until the sauce is reduced to 1/2 cup and is slightly syrupy, about 40 minutes.
Set a fine-mesh sieve over the jar of a blender. Strain the sauce, pressing on the solids to extract the liquid. With the blender on, slowly pour in the remaining 3/4 cup of oil until combined.
Light a grill or heat 2 grill pans. Lightly brush the tuna steaks with oil and season with salt and pepper. Grill the steaks over moderately high heat, turning once, until medium rare, about 6 minutes. Transfer the steaks to plates, spoon the sauce on top and serve with the wasabi on the side.