Sweat the onion in the olive oil until translucent, add rice and gently stir to coat the grains, keep stirring for about one minute to toast the grains. Add wine and reduce heat, add 1/3 quart of stock, stir until most of the liquid is absorbed, repeat the process until the rice is cooked through, season with salt, remove from heat, add pumpkin and cheese, and season. Spread on a lined sheet. Cool. Once cool, mold mixture into 1.5 tablespoon-sized balls. Set up the standard breading procedure in bowls (flour, beaten egg with milk, crumbs). Dredge each arancini ball in flour to evenly coat all sides. Tap off excess flour, roll in egg wash, roll in panko bread crumbs, covering it completely. Refrigerate until ready to fry.