Melt 1/2 Tbs. of the butter in a 12-inch skillet over medium-high heat. Add half of the spinach and cook, tossing with tongs, until just wilted, about 2 minutes. Transfer with tongs to a colander set over a bowl; let drain and cool slightly. Repeat with the remaining spinach (you don’t need to add more butter). Squeeze handfuls of the spinach to release as much liquid as possible.
Discard any liquid in the skillet. Melt the remaining 2 Tbs. butter over medium heat and then add the leeks and a pinch of salt. Cook until softened but not browned, about 5 minutes. Add the garlic and cook, stirring, for 1 minute more. Add the wine, raise the heat to medium high, and cook until almost evaporated, about 2 minutes. Add the cream and simmer until it’s thickened and coats the back of a spoon, about 2 minutes. Season with 1/2 tsp. salt, a generous grind of pepper, and the nutmeg. Stir in the cheese and gently fold in the spinach. Keep warm.
Pat the scallops dry and remove the side muscle if still attached. In a 12-inch nonstick skillet, heat 2 tsp. of the oil over medium heat until shimmering hot. Season the scallops with salt and pepper. Add half of the scallops to the pan and cook, undisturbed, until browned on the bottom, 2 to 3 minutes. Flip and continue to cook until just opaque in the center, about 2 minutes more. Transfer to a plate and tent with foil to keep warm. Repeat with the remaining 2 tsp. of oil and the scallops. Serve the scallops over the spinach.