Preheat the oven to 350 degrees F (175 degrees C). Line a baking sheet with aluminum foil and spray with cooking spray.
Combine beef, bread crumbs, egg, 3 tablespoons mint, 2 tablespoons Greek yogurt, salt, garlic powder, cumin, and pepper in a large bowl. Form into 24 meatballs and place on the prepared baking sheet.
Bake in the preheated oven until meatballs are no longer pink in the center, about 20 minutes.
Heat oil in a large skillet over medium-high heat. Cook onion until translucent, about 3 minutes. Add flour. Add beef broth gradually and cook until sauce starts to thicken, about 2 minutes. Add meatballs. Whisk in 1/4 cup Greek yogurt and 2 tablespoons mint. Serve over couscous.