I grew up eating arancini with my dad at a local Italian cafe! I remember how excited he was when he realized we could get them locally, and I instantly fell in love with them and started making them with my mom every year around the holidays. They were huge and always filled with melty mozzarella. Here, I'm using fontina for a bit of nuttiness and I've added prosciutto because, well, I can never have too much prosciutto! These are great for using up leftover risotto and turning it into a fabulous appetizer.
Line a baking sheet with parchment paper. Heat the stock in a medium saucepan over low heat to simmering.
Heat the olive oil in a medium straight-sided skillet over medium heat until hot. Add the onion and cook, stirring, until it begins to soften but does not color, about 4 minutes. Add the rice and toss to coat in the oil. Add the white wine and cook until completely absorbed, about 2 minutes.
Ladle in enough stock to just cover the rice. Adjust the heat so the liquid is simmering evenly. Cook, stirring occasionally, until the liquid is almost absorbed. Add the stock twice more in this manner (you may not use all of the stock) until the risotto is creamy and al dente, 16 to 18 minutes from the first addition of liquid. Off the heat, stir in the Parmesan and butter until smooth. Spread the mixture on the prepared baking sheet and let cool completely.
Once the risotto is cooled, scrape it into a large mixing bowl. Beat 1 egg and add to the bowl along with the fontina and prosciutto (separating any clumps of prosciutto with your fingers). Stir to combine. Line the baking sheet with a clean sheet of parchment. Scoop about 1/4 cup of the risotto into your hands and roll into a ball. Place on the baking sheet. Repeat with the remaining risotto, to get about 18 balls. Refrigerate until just firm, about 30 minutes.
Meanwhile, heat about 2 inches vegetable oil in a medium Dutch oven to 350 degrees F. Break the remaining 2 eggs into a shallow bowl and beat with a pinch of salt and a tablespoon of water. Spread the flour and breadcrumbs in 2 separate shallow bowls.
Dredge the balls in the flour, then the egg and finally the breadcrumbs, taking care to coat them completely. Return them to the baking sheet.
Fry the balls in 2 or 3 batches until deep golden brown, 4 to 5 minutes per batch, letting the oil return to temperature before adding another batch. Drain on a paper towel-lined baking sheet. Serve warm or at room temperature, sprinkled with Parmesan and chopped parsley.