To make the risotto, in a bowl, toss the mushrooms with the lemon juice. 

In a large saucepan over medium-high heat, melt 2 tablespoons of the butter. Add the mushrooms and cook, stirring occasionally, until they release their juices and begin to brown. Stir in the parsley and transfer to a small bowl. Set aside.

In the same pan over medium heat, melt 2 tablespoons of the butter with the olive oil. Add the onion and cook, stirring, until it softens and begins to brown, 5 to 7 minutes. Add the rice and stir for 3 to 4 minutes to coat the grains with the oil.

Add the mushrooms and wine, stirring constantly until the liquid is absorbed. While stirring constantly, add the hot stock a ladleful at a time (being sure each addition is absorbed before adding more). Continue until you have used all of the stock and the rice is al dente (tender but firm to the bite), about 25 minutes.

Stir in the remaining 2 tablespoons butter and the Parmesan, and season with salt and black pepper. Let cool to room temperature.

To make the arancini, preheat the oven to 375°F. Line a baking sheet with parchment paper or grease with olive oil.

in a large bowl, beat 2 of the eggs. Add the cooled risotto, Parmesan, sage, parsley, salt and cayenne and stir well to combine.

In a small bowl, whisk together the remaining 2 eggs and the olive oil. In another small bowl, stir together the bread crumbs and plenty of black pepper.

Dampen your hands and shape the risotto mixture into 2-inch balls. Insert a mozzarella cube into the center of each one. Dip each ball in the egg-olive oil mixture and then in the bread crumb mixture to coat. 

Transfer to the prepared baking sheet and bake until golden brown underneath, 15 to 20 minutes. Turn the balls over and bake until golden brown on both sides, 10 to 15 minutes more. Remove from the oven and let the arancini cool for a few minutes. 

To make the blue cheese aioli, in a small bowl, whisk together the garlic, whole-grain mustard, Dijon mustard, mayonnaise and blue cheese until combined.

To serve, drizzle the arancini with the truffle oil and serve with the blue cheese aioli for dipping.