Cook gyro slices according to package directions. Set aside.
In 10-inch omelet pan or skillet over medium heat, Add spinach, 1/4 cup of the remaining onions, parsley, mint, salt, and pepper. Cover. Cook, stirring occasionally, until spinach is tender. Set aside and keep warm.
Beat 3 eggs and 2 tablespoons water in small bowl until blended.
Melt 1 tablespoon butter in a 7- to 10-inch nonstick omelet pan or skillet over medium-high heat until hot. Pour in egg mixture. Mixture should set immediately at edges.
Gently push cooked portions from edges toward the center with inverted turner so that uncooked eggs can reach the hot pan surface. Continue cooking, tilting pan and gently moving cooked portions as needed.
When top surface of eggs is thickened and no visible liquid egg remains, fill omelet with spinach and gyro mixture. Let it heat thoroughly, with pancake turner fold omelet in half and slide onto plate.
Garnish with tomatoes and mint, if desired.