Preheat the oven to 425 degrees F (220 degrees C). Roll out pizza dough and cut out circles about 3-inches in diameter. Place mini crusts on a baking sheet. Bake in the preheated oven until slightly golden, about 5 minutes. Meanwhile, heat olive oil in a large skillet over medium-high heat. Cook and stir spinach in the hot oil until cooked down and softened, 5 to 7 minutes. Let cool for about 5 minutes; remove most of the liquid by squeezing. Cover mini pizza crusts lightly with pesto, leaving a small, plain border. Top each with sauteed spinach, Gorgonzola cheese, olives, and onion slices. Bake in the preheated oven until cheese melts, about 5 minutes.