How to Make It
In a bowl, whisk the vinegar with the mustard. Slowly whisk in the olive oil and season with salt and pepper.
In a large nonstick skillet, melt 1 tablespoon of the butter until foamy. Add the apple wedges and cook over moderately high heat until softened and golden, about 4 minutes; transfer to a plate. Melt 1 more tablespoon of the butter in the skillet. Add the potato slices and cook until tender, about 5 minutes; add to the apples.
Melt the remaining 1 tablespoon of butter in the skillet. Season the scallops with salt and pepper and add them to the skillet in a single layer. Cook over moderately high heat, turning once, until golden, 5 to 6 minutes. Transfer the scallops to the plate with the apple wedges. Add half of the vinaigrette to the skillet and cook, scraping up any browned bits, just until heated through.
In a bowl, toss the salad greens with the remaining vinaigrette. Arrange the greens on 4 plates. Spoon the apples, potatoes and scallops over the salads and drizzle the pan juices all around.