Make the Rice
In a large saucepan over medium-high heat, place the rice, saffron, and 1 1/2 cups water and bring to a boil.
Cover, reduce heat to low, and let simmer about 15 to 20 minutes or until all of the water has been absorbed.
Remove lid, stir in grated Parmigiano, butter, salt, and pepper, to taste.
Spread the rice out on a large plate or baking dish to cool completely to room temperature.
Make the Meat Sauce and Filling
Heat the olive oil in a small saucepan over medium-high heat. Add the onions, carrots, and celery and sauté, stirring often, until softened and the onions are translucent - about 8 to 10 minutes.
Add the ground beef and pork and fry, stirring often, until browned - about 5 to 8 minutes.
Add the wine and let cook until the alcohol aroma has reduced - about 1 minute.
Stir in the tomato paste and tomato purée, reduce the heat to medium-low, and cook, stirring occasionally - about 10 minutes.
Stir in meat sauce.
Add the peas and continue to simmer the sauce for another 8 to 10 minutes, or until the peas are tender and the sauce is thickened. It should not be too liquidy.
Transfer the filling to a bowl and set aside to let it cool.
Once the rice and filling are completely cooled, start shaping your rice balls.
Place 1 heaping tablespoon of rice in the palm of one hand, then use your fingers and thumb to shape it into a hollow bowl shape.
Place about 1 teaspoon of the filling in the center and 1 to 2 small cubes of diced fresh mozzarella (if using).
Then gently close the rice around the filling to form either a round ball shape or a cone/pear shape.
When all of your arancini have been formed, whisk together the flour, eggs, 1/2 cup of water, and a pinch of salt in a shallow bowl until smooth.
Spread the bread crumbs in a plate or baking dish.
Gently roll each ball first in the egg-flour-water-salt mixture, letting any excess drip off.
Then roll them in the breadcrumbs until evenly coated.
At this point, you can refrigerate your arancini for 20 to 30 minutes to let them firm up if they seem a bit too loose or liquidy. If not, you can go directly to frying.
Heat about 2 inches of vegetable frying oil to 360F.
Fry your arancini in batches of just 2 to 3 at a time, being careful not to overcrowd the pot until they are evenly golden-brown - about 3 minutes.
Transfer them to a paper-towel-lined plate to drain.