1| Put stock in a large saucepan and bring to the boil. Stir in rice, cover, reduce heat to low and simmer for 10 minutes. Remove from heat. Set aside, covered, for 15 minutes. Transfer to a bowl.
2| Meanwhile, heat oil and butter in a medium pan on a medium heat. Add onion, garlic, bacon and mushroom. Cook, stirring occasionally, for 8-10 minutes or until golden. Add onion mixture and cheeses to rice, and stir to combine. Season with salt and pepper. Set aside to cool, then refrigerate for 2 hours.
3| Shape tablespoons of mixture into balls. Coat balls with flour, dusting off excess. Dip into combined egg and milk, then coat with breadcrumbs. Cover and refrigerate until ready to cook.
4| Heat enough oil in a large saucepan to come one-third of the way up the side of the pan. Cook arancini in batches, turning, for 4-5 minutes or until golden. Drain on paper towel. Serve immediately.