Melt the butter in a large saucepan over medium heat. Add the leeks and garlic and cook for a minute or two, or until the leeks are lightly browned. Sprinkle with the flour and stir until fully saturated. Slowly add in the milk and clam juice, stirring or whisking constantly until fully incorporated. Add the lemon juice, zest, dill, and peas. Bring to a light simmer and then reduce the heat to low until the sauce thickens.

Meanwhile, pat the sea scallops dry and lightly season with salt and pepper. Heat 2 tablespoons of butter in a frying pan over medium heat. Add the scallops. Cook 2 to 3 minutes per side until lightly golden and cooked through. Spoon the butter over them as they cook.

Dish mashed potatoes onto plates and divide the scallops evenly among them. Drizzle the white gravy with leeks and peas over the top and garnish with watercress, dill, and lemon slices, as desired.

Serve immediately. Enjoy!