Cook spinach in a medium pan over medium heat until spinach is wilted.
Place all ingredients in a large bowl and mix until completely combined.
Tip: Cover and refrigerate combined ingredients for 1 hour before forming into patties if the ingredients are not sticking together when rolling into portions.
With clean hands, divide meat into 4 equal portions (about 6 oz each). Form burgers into ¾-inch thick patties (be sure to smush them fairly flat so that they will cook evenly without drying out).
Place patties on a hot grill coated with cooking spray or in a large skillet covered with cooking spray. Cook burgers for about 5-6 minutes per side until burgers are fully cooked.
Alternatively, you can cook in the oven on a broiler pan at 400 for 30 minutes, turning burgers at 15 minutes.