How to Make It
In a large saucepan, melt 2 tablespoons of the butter. Add the onion and cook over moderate heat, stirring, until lightly browned, 7 minutes. Add the rice and cook, stirring, until well coated with butter. Add the white wine and saffron, season with salt and black pepper and cook, stirring, until the wine is absorbed, 2 minutes. Add the warm chicken stock 1/2 cup at a time and cook, stirring constantly between additions, until it is absorbed. The risotto is done when the rice is al dente, 25 minutes total. Stir in the grated cheese, transfer to a bowl and let cool.
Melt the remaining 1/2 tablespoon of butter in a small saucepan. Add the 1/2 tablespoon of flour and whisk constantly over moderate heat for 1 minute. Add the milk and cook, whisking, until thickened. Season with the nutmeg, salt and black pepper and transfer to a bowl to cool completely. Stir in the mozzarella, pistachios and peas.
Line a large baking sheet with wax paper. Put the eggs, panko and flour for dusting in 3 shallow bowls. Using lightly moistened hands, shape the rice mixture into 16 equal balls. Working with one ball at a time, make an indentation in the center with your finger and press the sides to make the hollow larger. Spoon a scant tablespoon of the pistachio filling into the hollow and press the risotto around the filling to enclose it. Transfer the ball to the baking sheet. Repeat with the remaining risotto and filling. Dust the arancini with flour, tapping off the excess. Coat them with the egg and roll in the panko.
In a large, deep skillet, heat 1 inch of vegetable oil to 350°. Fry the arancini over moderate heat, turning occasionally, until golden and heated through, 8 minutes. Drain the arancini on paper towels and serve hot.