Pat scallops dry. Heat a large frying pan (not nonstick) over high heat until a drop of water sizzles when flicked onto it. Add grapeseed oil, then scallops; season with 1/4 tsp. salt. Sauté scallops without moving them until browned and easily turned, 3 minutes. Turn and brown other side, 2 minutes. Transfer to a plate.
Whisk lemon zest and juice with olive oil and 1/2 tsp. salt.
Cook peas in a saucepan of salted boiling water until just tender, 2 minutes for fresh, 30 seconds for frozen; drain and rinse with cold water.
Toss mâche, peas, and prosciutto with 2 tbsp. dressing. Arrange on a platter with scallops. Drizzle with dressing to taste and serve immediately.
*Dry-packed sea scallops smell fresh and are slightly sticky with no liquid oozing from them; find them at good seafood shops.