Finely chop the onion and set it aside.

Cut the red bell pepper and carrots into small pieces.

Mix the ground meat with the egg and breadcrumbs in a bowl. Make sure everything is well combined for a firm texture.

Season the mixture with salt and pepper to taste.

Roll meatballs from the ground meat mixture.

Heat a knob of butter in a pan over medium heat.

Brown the meatballs all over in the heated pan. Make sure they are evenly browned.

Remove the meatballs from the pan and let them rest briefly on a plate.

Add the chopped onion to the same pan and sauté until translucent.

Add the chopped bell pepper and carrots to the pan and cook for a few minutes until slightly softened.

Add the passata to the pan and stir everything well.

Let the sauce simmer gently for 5 minutes so the flavors can develop.

Carefully return the browned meatballs to the sauce and let them cook over low heat for another 15 minutes.

Adjust the seasoning of the sauce with extra pepper, salt, and any other herbs or spices you prefer to enhance the flavor.

Bake the croquettes in the air fryer according to the package instructions until golden brown and crispy.