Preheat your grill or grill pan to medium-high heat
In a large skillet, sauté the spinach and garlic until wilted
Remove from heat and stir in the parmesan cheese and lemon zest
Lay out the flank steak slices on a cutting board
Spread a thin layer of the spinach mixture over each slice of steak
Roll up each slice tightly into a pinwheel shape
Brush each pinwheel with olive oil and season with salt and pepper
Grill the pinwheels for about 10 minutes, turning occasionally, until cooked to your desired doneness
Remove from the grill and rest for a few minutes before slicing into rounds using a sharp knife
Serve hot, and enjoy!