Preheat your oven to 400°F (200°C).
In a large bowl, combine cooked arborio rice, grated Parmesan, and cubed mozzarella.
Add the beaten egg, salt, and pepper; mix everything together until well combined.
Shape the mixture into golf ball-sized spheres.
Dust each ball in flour, then dip in remaining beaten egg, and finally coat with breadcrumbs.
Place the arancini on a baking sheet lined with parchment paper.
Spray the arancini lightly with cooking spray or drizzle with olive oil.
Bake for 25-30 minutes, turning halfway through.
Let them cool slightly before serving.
