In a large pot of boiling water, cook the broccoli rabe until crisp-tender, about 2 minutes; drain and squeeze out any excess moisture.
Meanwhile, preheat a grill or grill pan over high heat. Coat the tuna with the vegetable oil and season with salt and pepper. Grill the tuna, turning once, for 4 to 6 minutes for medium-rare. Transfer to a cutting board. Cut the tuna in half, then thinly slice against the grain into 18 pieces; tent with foil.
In a small bowl, combine the honey, lemon juice and wasabi paste. In a large, deep skillet, heat the olive oil over medium-high heat. Add the garlic and cook, stirring, until softened, 2 to 3 minutes. Add the broccoli rabe and toss to heat through. Stir in the wasabi dressing; season with salt. Divide the broccoli rabe among 6 small plates and arrange 3 tuna slices on each; sprinkle with the sesame seeds.