Add the ground turkey, chopped spinach, feta, garlic, breadcrumbs, Worcestershire sauce, black pepper, and olive oil to a large mixing bowl.
Gently mix until the ingredients are evenly distributed. Avoid overworking the turkey, as this can make the cooked patties dense.
Divide the mixture into 4 equal portions. Shape each portion into a patty approximately 4 inches wide and 1 inch thick.
Place the patties on a parchment-lined plate. Refrigerate for about 1 hour, or chill them in the freezer for approximately 20 minutes if you are short on time.
Preheat the grill to medium-high heat and thoroughly oil the grill grates to help prevent sticking.
Grill the Turkey Burgers for about 4–5 minutes per side, or until completely cooked through. Ground turkey should reach an internal temperature of 165°F (74°C) in the center.
Place the cooked patties on toasted hamburger buns and finish with mayonnaise, cucumber slices, and thinly sliced red onion. Serve immediately.
Notes
For a lighter serving option, skip the hamburger buns and serve the patties over a crisp green salad with cucumber, tomatoes, and red onion.