Heat a large stainless steel skillet with 2 turns of the pan of EVOO, about 2 tablespoons, until it's smoking hot.
Season the scallops with salt and pepper, and add them to the hot skillet one by one. Make sure you don't overcrowd the pan.
Cook the scallops until golden in color, about 3-4 minutes on each side, depending on how big of a skillet you have. You might need to cook them in 2 batches.
While the scallops are cooking, lay the prosciutto on a serving platter, completely covering the surface from edge to edge. In a large mixing bowl, combine the sliced portabellos, arugula, celery, onion, lemon juice, salt and pepper and a good drizzle of EVOO.