If using broth that doesn't gel when refrigerated, measure out broth and sprinkle gelatin over the surface. Set aside.
Add butter and oil to a large dutch oven or saucepan and heat over medium-high until butter is melted and just starting to bubble.
Add onion and stir until they begin to sweat and turn translucent, about 4 minutes.
Add rice and continually stir until rice is coated in oil and slightly toasted.
Add wine and cook until almost completely evaporated, about 2 minutes.
Slowly stir in two cups bone broth. Continue stirring until rice has absorbed most of the liquid.
Stir in one more cup of bone broth and simmer, stirring often, until almost all the liquid is absorbed.
Stir in additional water, if needed, 1/4 cup at a time. Continue cooking and stirring until rice is tender, just past al dente, and the consistency of a thick porridge. Season with salt and pepper.
While your rice finishes cooking, make the veloute sauce. Melt butter in a small saucepan over medium-high heat until foamy bubbles appear.
Quickly whisk in two tablespoons flour until it forms a paste.
Lower the heat to medium-low and cook two to three minutes, stirring consistently, until flour is cooked but not browned.
Whisking continually, add in 1/4 cup milk and 3/4 cup bone broth and stir until smooth.
Bring to a simmer and continue to cook, stirring continually for about four minutes, or until sauce is thick enough to coat the back of a spoon.
Remove from heat, season with salt and pepper and once it’s cooled slightly, whisk in eggs.
Combine rice and veloute in a large bowl and stir until rice is evenly coated with sauce.
Line two large sheet-pans with parchment paper and spread rice mixture in a thin layer over the parchment.
Place in the refrigerator to cool completely, approximately one hour.
Transfer rice to a large bowl and line the baking sheets with a fresh sheet of parchment paper.
Grab 1/4 cup of rice mixture and form a flat disk with your hands. Don’t worry about the rice sticking to your hands…it’s all part of the fun!
Place a cube of diced mozzarella into the center of the disk. Mold the rice around the cheese to form a ball.
Continue with remaining rice mixture until all Arancini are formed.
Set up "breading station" by mixing 1/4 cup water and 1/4 cup flour together in a shallow dish. Once you have a thick paste, add 1/4 cup more water and whisk until you have a thin "slurry". In another shallow dish, add bread crumbs. (I whizzed mine first in the Vitamix to make them extra fine.)
Take Arancini and roll in the water/flour slurry, then quickly roll in bread crumbs to coat completely and evenly.
Place on parchment paper and continue with the remaining Arancini until all are coated.
Heat 2-4 inches of oil in a heavy bottomed pot to 375 degrees F. Use a thermometer and watch temperature carefully!
One at a time, re-shape Arancini into a ball, if needed, and lower into oil using a slotted spoon. Cooking them in small batches, gently roll them around until all sides are evenly cooked and golden brown.
Remove from oil and drain on a paper towel-lined plate. Continue until all rice balls are cooked. Top oil if needed, but allow the temperature to return to 375 F before continuing to cook.
Serve Arancini as is, or with marinara dipping sauce.
Arancini can be refrigerated or frozen and reheated at 350 degrees F until heated through. (If frozen, defrost before reheating.)