Add all ingredients to a mixing bowl and mix with your hands until thoroughly combined.
Divide the mixture into 4 equal sized balls and smash into round patties, approximately 1 inch thick.
Transfer to a plate and refrigerate for at least 2 hours before cooking.
Cook on grill over medium heat to an internal temperature of 165°F, around 4-5 minutes per side.
Notes
For spinach I prefer to use fresh spinach and saute it in 1 tablespoon of olive oil for around 2 minutes until wilted. You can also use raw chopped spinach or frozen chopped spinach that has been thawed and thoroughly drained.