How to Make It
In a small saucepan, simmer the soy sauce, sake, mirin and shallot until the liquid is slightly reduced, 3 minutes. Remove from the heat; stir in the ginger. Slowly whisk in 1/4 cup of the oil. Season with salt and pepper.
In a steamer basket set in a large saucepan of simmering water, steam the Broccolini until tender, about 6 minutes. Transfer to plates.
Meanwhile, in a large nonstick skillet, heat the remaining 2 tablespoons of oil. Season the tuna with salt and pepper. Sear over high heat until golden brown but still rare within, about 30 seconds per side. Transfer to a paper towel–lined plate to drain. Slice against the grain and transfer to the plates. Drizzle with some of the vinaigrette and sprinkle with the sesame seeds. Serve with lemon wedges and the remaining vinaigrette.