Instructions

Adjust oven rack to middle position and heat oven to 300 degrees. Spray 8-inch square baking pan with canola oil spray. Whisk 8 eggs in large bowl until homogeneous. Whisk in ⅔ cup water, 2 teaspoons cornstarch, and ¼ teaspoon salt until well combined. Pour egg mixture into prepared pan and cover tightly with aluminum foil. Transfer to oven and bake until eggs are fully set, 30 to 35 minutes, rotating pan halfway through baking. 
Run plastic knife around edges of pan and, using dish towel or oven mitts, invert eggs onto cutting board (if eggs stick to pan, tap bottom of pan firmly to dislodge). Cut into 4 equal squares.
Arrange egg squares on greased rimmed baking sheet and top with 4 ham slices. Bake until ham is heated through, 1 to 3 minutes.
Meanwhile, combine ¼ cup Greek yogurt, 4 teaspoons mayonnaise, and 1 tablespoon Dijon mustard in bowl. Arrange egg squares on 4 toasted whole-grain bread slices, then top with 2 cups arugula, sauce, and remaining bread slices. Serve.To Make Ahead: Top the egg squares with ham and refrigerate for up to 3 days or freeze, wrapped tightly in plastic wrap, for up to 1 month. ­Refrigerate the sauce for up to 3 days. When ready to assemble a sandwich, microwave 1 egg square at 50 percent power until heated through, 1 to 2 minutes from ­refrigerated or 2 to 4 minutes from frozen. Top the egg square with 1 cup arugula. To make individual portions of sauce if needed, combine 1 tablespoon yogurt, 1 teaspoon mayonnaise, and ½ ­teaspoon mustard in a bowl.