Heat 4 1/2 teaspoons oil in a large nonstick frypan over medium high heat until hot but not smoking.
Add leek, cook until soft, 1 to 2 minutes; Stir in cabbage, and add stock.
Cook stirring occasionally, until cabbage is slightly soft, about 5 minutes. Stir in 1/2 teaspoon salt and season with fresh ground pepper; set aside.
Sprinkle scallops with 1/4 teaspoon salt and season with freshly ground pepper. Heat remaining 2 1/2 teaspoons oil in another large nonstick frypan, over medium high heat until hot but not smoking. Add scallops, cook, turning once , until dark golden, about 4 minutes per side.
Divide cabbage mixture between four plates and top each serving with 3 scallops, garnish with lemon slices.