Place stock in a large saucepan over high heat and bring to the boil. Reduce heat to low and keep stock hot.

Place mushrooms in the bowl of a food processor and process using the pulse function until finely chopped. Transfer to a bowl.

Heat olive oil in a large, deep heavy-based frying pan over high heat. Cook mushrooms for 6 minutes or until golden. Transfer to a bowl.

Add butter to same pan over medium heat. Cook onion, stirring, for 4 minutes or until golden. Add garlic and cook for 1 minute. Add rice and cook, stirring, for 2 minutes until well coated.

Add one-third of the hot stock. Cook, stirring, for 7 minutes or until almost absorbed. Repeat two more times with remaining stock. (This step should take approximately 20 minutes.) Stir through mushrooms and half the parmesan. Season to taste. Transfer risotto to a large, shallow roasting pan and freeze for 20 minutes or until chilled and firm.

Combine breadcrumbs, thyme and remaining parmesan in a shallow bowl. Place seasoned flour and beaten egg in two separate shallow bowls. Roll 1 tablespoon portions of risotto into 28 balls. Push a bocconcini half into the centre of each ball and mould risotto around to seal. Coat balls first in flour, then egg, then breadcrumb mixture.

Heat oil in a large, deep, heavy-based saucepan to 170°C. Cook arancini, in batches, for 4 minutes or until crisp and golden. Drain on paper towel.