Heat olive oil in a skillet over medium-low heat. Add the green onions and mushrooms and cook until soft and brown, about 5 minutes. Add the garlic and cook for 1 additional minute. Turn off heat and allow to cool slightly.
Add the turkey in a large bowl and add the mushroom mixture, salt and pepper. Mix until well and form into 6 patties.
Preheat a grill to medium-high heat. Spray lightly with cooking spray and grill the burgers until cooked through, 6-8 minutes per side.
Top the spinach with burgers, add tomato slices and serve immediately.