Make the Risotto Base:

1. Cook the onion:
   • Heat butter and olive oil in a skillet over medium heat. Add the onion and sauté until translucent.
2. Cook the rice:
   • Stir in the Arborio rice and toast for 1–2 minutes. Gradually add the warm vegetable stock, one ladle at a time, stirring frequently until the liquid is absorbed. Repeat until the rice is tender and creamy. Season with salt and pepper.
3. Cool the risotto:
   • Spread the cooked risotto on a baking sheet and let it cool completely before proceeding (refrigerate for faster cooling).

Prepare the Arancini:

4. Form the balls:
   • Take a small handful of cooled risotto and flatten it in your palm. Place a cube of mozzarella in the center, then form the risotto around it to create a ball. Repeat with remaining risotto and mozzarella.
5. Set up the breading station:
   • Place the flour, beaten eggs, and breadcrumbs in separate bowls. Roll each rice ball in flour, dip it in the eggs, and coat it with breadcrumbs.
6. Fry the arancini:
   • Heat vegetable oil in a deep skillet or pot to 350°F (175°C). Fry the rice balls in batches until golden brown and crispy (about 2–3 minutes per side). Drain on paper towels.

Serve:

• Serve warm with marinara sauce for dipping, or enjoy them plain!