Mix flour, yeast and salt in a bowl. Gradually add 1/2 cup warm (110°F) water until a soft dough forms. Divide dough into 4 portions. Using hands, roll dough into balls. Coat with cooking spray, cover with plastic wrap and set aside to rise, 30 to 45 minutes. When ready to cook, place each ball of dough on a piece of wax paper and flatten into a 4-inch circle. Coat grill with cooking spray and heat on high. Gently place dough onto grill and cook until dough puffs and underside stiffens (about 30 seconds); flip and reduce heat to low. Mix topping ingredients, then divide among crusts. Cover grill. Cook until cheese melts, 5 to 6 minutes more. Slice and serve.