Prepare the grill. Rinse shrimp; pat dry with paper towels. Place shrimp in a small bowl; add garlic, lemon zest, butter and salt. Toss to combine. Cover and let sit for 30 minutes.Combine spinach, romaine, tomatoes, cucumber, olives and red onion in a medium bowl; toss to combine. Set aside.Thread shrimp onto 4 8-inch skewers, leaving a 1/4-inch space between shrimp. Place skewers on the prepared grill and cook for 6-8 minutes or until shrimp are opaque, turning once halfway through grilling.For the vinaigrette: Combine all ingredients in container of food processor fitted with metal blade; cover and process until well blended.Arrange greens in large serving platter. Top with feta cheese and grilled shrimp. Drizzle with Greek Vinaigrette.