In a large skillet, heat the olive oil over medium heat. Add the diced onion and garlic, sautéing until fragrant, approximately 30 seconds. Incorporate the finely chopped mushrooms and cook for 3-5 minutes, allowing most of the moisture to evaporate.

Stir in the Arborio rice, ensuring it is well coated with the oil and mushrooms. Toast the rice for approximately 30 seconds, enhancing its nutty aroma.

Pour in the white wine, stirring continuously until it evaporates completely. This step introduces acidity to balance the richness of the risotto.

Gradually add the warm stock, one cup at a time, stirring frequently. Allow the rice to absorb each addition of stock before adding more. Continue this process for approximately 35-40 minutes or until the rice becomes tender yet maintains a slight bite. The mixture should appear creamy but not overly thick.

Once the risotto has finished cooking, remove it from heat. Stir in the cold butter and freshly grated parmesan cheese, ensuring both are fully melted and combined.

Transfer the cooked risotto to a shallow baking dish and spread it out evenly. Place it in the refrigerator for at least 2 hours to cool completely. This step is essential for shaping the arancini properly.

Once the risotto has fully cooled, take a small handful and flatten it in your palm. Place a cube of mozzarella cheese in the center and enclose the rice around it, forming a ball. Ensure the mozzarella is fully covered to prevent leakage during frying. Repeat with the remaining risotto and cheese cubes.

Set up three bowls:

One with all-purpose flour

One with the beaten eggs

One with panko breadcrumbs

Roll each arancini ball first in the flour, ensuring an even coating. Then, dip the ball into the beaten eggs and finally coat it thoroughly with panko breadcrumbs, pressing gently to help the crumbs adhere.

Heat oil in a deep fryer or large pot to 350°F (175°C). Fry the breaded arancini in batches for approximately 5 minutes or until golden brown and crispy. Avoid overcrowding the oil, as it can lower the temperature. Once fried, place the arancini on a plate lined with paper towels to absorb any excess oil.

Arrange the mushroom arancini on a serving platter with warm marinara sauce. Garnish with freshly chopped basil and additional parmesan cheese for an elegant presentation.