Here's the official spanakopita burger recipe from Rachael Ray:
1 Tbsp EVOO
1 tsp butter
2 garlic cloves, chopped
1 onion, half chopped, the rest for topping the burger
a 10-oz box frozen spinach, defrosted
1 tsp dried oregano
1/4 lb. feta crumbles
1 1/3 lbs ground chicken or turkey
1 Tbsp grill seasoning if you want it
Saute the onion and garlic in the oil and butter over medium heat for about 5 minutes. Remove to a bowl to cool.
Wring the defrosted spinach dry. Throw everything into the bowl, mix together, and form four patties from this. Raise the heat to medium high, add the patties, cook 6 minutes on each side (or grill them, whatever).
She tops it with sliced cucumbers, tomatoes, sauteed onion slices, shredded arugula and hot peppers, then slathers the top bun with a mix of roasted red peppers and olives.