To make the Topping: Combine cucumber, tomatoes, green onions, lemon juice and oil in a bowl. Stir in feta cheese, if using. Season with salt and pepper and keep ready.
To make Croquettes: In a mixing bowl, combine chickpea flour, cumin, chili powder and salt. Whisk in ¾ cup hot water. Stir in the other ingredients and season with salt and pepper.
Heat 1tbsp oil in a nonstick skillet; scoop about 3tbsp dollops of chickpea mixture into skillet, and reduce the heat to medium-low. Cook 3~4 minutes or until golden. Flip and cook 3~4 minutes more. Repeat with remaining chickpea mixture.
Serve each croquette topped with the topping and drizzle some of tzatziki sauce.