If you are using fresh spinach, steam it until it is fully wilted, then squeeze out as much of the liquid as possible. If using frozen, thaw it and drain it as thoroughly as possible.
Combine all ingredients except for breadcrumbs in a medium mixing bowl.
In a shallow dish or bowl, combine breadcrumbs with salt and pepper, to taste.
Roll mixture into balls (like meatballs) and coat in breadcrumbs. Place on a parchment covered baking sheet.
At this point, you may store in the fridge until you are ready to cook, or flash freeze them/store in the freezer for up to 6 months.
When you are ready to cook, drizzle arancini with a little olive oil and bake at 375 for 20 minutes on a parchment covered baking sheet.
Serve with marinara sauce for dipping, if desired.