Fill a medium saucepan with water and bring to a boil. Drop the peaches into the water for 10 seconds. Remove and rinse under cold water. Use a small knife to slip off the skins. Cut each peach into 8 wedges; set aside.
Combine the ricotta and 1/4 cup of the sugar; set aside. In a small bowl, combine the cinnamon, nutmeg, and remaining sugar; set aside.
Lightly flour the back of a large baking sheet. Roll out the pizza dough to 1/4 inch thick. Place the dough on the back of the baking sheet and brush it with half the butter. Preheat a gas grill to high; adjust the temperature to low after 15 minutes. (If cooking over charcoal, allow the coals to burn until covered with gray ash.)
Using the baking sheet like a big spatula, flip the dough directly onto the grill. Cook until the underside of the dough begins to brown, 4 to 6 minutes. (The crust may bubble.) Using tongs, slide the dough back onto the baking sheet, then flip the ungrilled side onto the grate. Brush the cooked side of the dough with the remaining butter. Arrange the peaches on top. Sprinkle with the pepitas and spiced sugar. Cover grill and cook for 8 to 12 minutes or until the underside of the crust browns.
Slide onto the baking sheet. Serve warm with the sweetened ricotta.
Rainy-Day Method:Preheat oven to 375° F. Brush half the butter onto a baking sheet. Prepare the sweet ricotta mixture as above. Prepare the dough as above and place it on the baking sheet. Bake 12 to 15 minutes. Remove from oven and top with the peaches, pepitas, and spiced sugar. Bake 10 more minutes or until the crust is golden.