Prepare the dough and a start medium-hot fire in the grill. 

Steam the spinach until just wilted, about 3 minutes. Drain well. Taking a handful of spinach at a time, squeeze out as much water as possible. Coarsely chop.

To make two large pizzas: Divide the dough into 2 balls. Lightly oil the grill rack. Stretch 1 ball of dough to fit the oiled grill rack. Place the grill rack over the fire, cover the grill, and grill the pizza until the bottom is browned, about 5 minutes. Remove the grill rack from the grill. Brush the top side with olive oil, then flip the pizza over. Brush the dough with olive oil and scatter 1 Tbsp. of the garlic over the browned crust. Sprinkle with half of the Parmesan and top with half of the spinach, half of the feta, and half of the onion. Return the pizza, still on the rack, to the grill. Cover the grill and cook for 5 to 8 minutes, checking the crust after 5 minutes. If at any point the pizza seems to be cooking too quickly, reduce the heat on a gas grill or move the pizza to a cooler spot to avoid scorching. 

Remove the pizza and then repeat with remaining ingredients.

*From The Vegetarian Grill, by Andrea Chesman.