FOR SNAIL BUTTER: Finely chop parsley and garlic together. Place in a bowl, add butter and zest, and mash together with a fork until well combined.
Place mache in a large bowl. Set aside.
FOR SCALLOPS: Generously season scallops with salt and pepper. Heat a large cast-iron pan over high heat until quite hot. Add oil and then scallops, slowly feeding them into pan, being careful not to crowd pan or lower the temperature of it; as scallops form a crusty brown sear on the bottom, 3 to 4 minutes, move them on top of each other and add more to the pan. When all scallops are browned on one side, remove pan from heat and dot scallops with snail butter.
Place skillet in oven for 5 minutes to melt butter and cook scallops through.
Transfer scallops and butter to bowl with mache, add lemon juice, toss, and serve immediately, with crusty bread to sop up buttery juices.
Makes 4 main-course servings; 6 small plates