Heat butter in a pan over medium heat and sauté chopped spinach until wilted. Let it cool slightly.
In a bowl, combine cooked Arborio rice, sautéed spinach, Mozzarella cheese, Parmesan cheese, and beaten egg. Season with salt and black pepper. Mix well.
Shape the mixture into small balls and roll in panko breadcrumbs until fully coated.
Heat vegetable oil in a deep pot to 350°F. Fry rice balls in batches until golden brown and crispy.
In a small bowl, whisk together mayonnaise, minced garlic, lemon juice, and paprika for the aioli sauce.
Serving
Serve the rice balls warm, accompanied by the garlic aioli for dipping.
Notes
You can substitute baby spinach with kale or swiss chard for a different flavor profile.