In small bowl, stir together sour cream, cucumber, 1/4 cup onions, and garlic powder until well combined. Cover and chill.
In 10-inch omelet pan or skillet over medium heat, cook beef until no longer pink. Drain. 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. See side bar for omelet recipe.
Fill half of each omelet with 1/2 cup beef mixture. With pancake turner, fold omelet in half and slide onto plate. Garnish with tomatoes and mint, if desired.