In a small bowl, stir together ketchup and Dijon mustard; reserve in the refrigerator.
In a medium bowl, stir together ground turkey, black olives, oregano, and garlic powder. Salt and pepper to taste.
Using a spoon, combine ground turkey mixture and shape into 4 patties. Grill over medium heat for 5 to 6 minutes per side, or until cooked through.
Place baby spinach leaves on the bottom halves of Onion Hamburger Buns. Place the turkey burgers on top and garnish with slices of tomato. Serve with tomato sauce on side, if desired.
Notes
Try this tasty recipe for a Bistro Turkey Burger, which is not only packed with grains but is also under 500 calories.