Shrimp tossed in dense Greek yogurt, mustard, garlic, and ketchup makes a delicious centerpiece for a leafy salad. Making remoulade with yogurt instead of mayonnaise cuts the calories by more than half.
Prepare an ice-water bath. Bring a stockpot of water to a boil. Reduce heat to low. Add shrimp, and poach until just cooked through, about 2 minutes. Transfer to ice-water bath. Let stand until cool. Drain. Peel and devein shrimp. Refrigerate.
Meanwhile, stir together yogurt, scallions, mustard, ketchup, capers, lemon juice, garlic, and cayenne. Toss shrimp in remoulade to coat; refrigerate for 30 minutes. Divide lettuce evenly among plates, and top with shrimp. Garnish with lemon wedges.
Make ahead: Tossed shrimp can be refrigerated for up to 4 hours.