Tomato and Cucumber Sandwiches with Herbed Feta Butter

Rinse onion slices beneath cold running water for 30 seconds (to remove sharp onion flavor); drain, pat dry with paper towels, and set aside. Combine cream cheese, butter, feta, mint, and chives in a bowl; set aside. Using a 2 1/2-inch round biscuit cutter, cut bread slices into rounds. Spread cheese mixture on bread rounds. Place chopped tomatoes on 8 bread rounds. Arrange 2 to 3 overlapping cucumber slices on remaining bread rounds. Sprinkle with salt. Top sandwiches with a small amount of sliced onion and sprouts. Arrange on serving platter.