In a pot of boiling salted water set over high heat, cook the haricots verts for 3 to 4 minutes, until tender. Drain and place them in the center of a large, warm serving platter. Cover and keep warm.
Season the halibut on both sides with salt and pepper. In a 12-inch saute pan, heat the oil over medium-high heat. Cook the fish for about 3 minutes, until lightly browned. Turn, reduce the heat to medium, and cook about 4 minutes longer, until the fish is opaque in the center and browned on both sides. Put the fish on the platter with the haricots verts.
Deglaze the pan with the white wine and lemon juice. Swirl in the butter gradually to make a rich sauce, then add the scallions, capers, and diced tomato. Pour the sauce over the fish and serve.