Heat the olive oil in a large pan over medium heat. Add the diced onion and garlic, then sauté for 30 seconds to 1 minute, until fragrant and slightly softened.

Add the finely chopped mushrooms and cook for 3 to 5 minutes, stirring frequently, until most of the moisture has evaporated.

Season with salt and add the Arborio rice. Stir for about 30 seconds to lightly toast the rice.

Pour in the white wine and continue stirring until the wine has mostly evaporated, about 2 minutes.

Begin adding the stock, 1 cup at a time, stirring frequently and allowing the liquid to absorb before adding more. Continue this process until the rice is tender and creamy. This should take approximately 35 to 40 minutes, though the total amount of stock needed may vary.

Once the risotto is fully cooked, stir in the cold butter and grated Parmesan cheese until smooth and well combined.

Transfer the cooked risotto to a long baking dish or shallow pan and spread it into an even layer.

Refrigerate for approximately 2 hours, or until completely cooled and firm enough to shape.

Cut the mozzarella into 1-inch cubes.

Take a small handful of the cooled risotto and place a cube of mozzarella in the center. Shape the risotto around the cheese to form a ball. Add more risotto if needed to fully enclose the mozzarella. Repeat with the remaining risotto.

Prepare a breading station with three separate bowls: one for the flour, one for the beaten eggs, and one for the panko breadcrumbs.

Roll each risotto ball in the flour, then dip it into the egg, and finally coat it thoroughly in the panko breadcrumbs. Press lightly to ensure the breadcrumbs adhere well.

Heat oil in a deep pan or fryer to 350°F.

Fry the arancini for about 5 minutes, or until golden brown and crisp on the outside. Work in batches if necessary to avoid overcrowding the pan.

Remove the arancini and place them on a paper towel-lined plate or wire rack to drain briefly.

Spoon marinara sauce onto a serving plate. Arrange the arancini over the sauce, then garnish with fresh basil and additional Parmesan cheese, if desired. Serve immediately.