Bring a large pot of salted water to a boil. Preheat a grill or grill pan over high heat. Season the tuna steaks with salt and pepper to taste, and spray them lightly with cooking spray. When the grill is hot, add the tuna and cook for 1 1/2 minutes per side for medium-rare. Transfer the tuna to a platter and allow it to rest, uncovered, for 5 minutes. Meanwhile, cook the haricots verts in the boiling water until they are just tender, about 3 minutes; drain. In a medium bowl, whisk together the lemon juice and zest, garlic, and wasabi paste. Add the haricots verts, scallions, and sesame seeds. Toss to coat, adding salt and pepper to taste. Thinly slice the tuna. Fan each portion onto each of 4 plates. Pile a mound of dressed haricots verts on top of the tuna, and serve.