In a small mixing bowl, combine breadcrumbs and milk and set aside.
Sauté the onion in butter and olive oil on low heat for about 10-15 minutes, until the onion is soft and starts browning slightly.
Place the ground chicken in the food processor and add the breadcrumbs, sautéed onion, egg, parsley, garlic, salt and pepper. Pulse this mixture until all the ingredients are evenly combined.
Put oil in a large skillet over medium heat.
Scoop ⅛th of a cup of the meatball mixture and roll it into a ball. You may need to wet your hands, because the mixture is sticky. One by one, roll the meatballs and add them to the oil.
Cook, turning as necessary, until nicely browned all over for about 10 minutes. You don't need the meatballs to cook through completely because you'll cook them in marinara sauce in a later step. (Depending on the size of your skillet, you may need to work in batches. I have a 10-inch skillet and I cooked these meatballs in 2 batches, 12 at a time).
Once the meatballs are cooked, transfer them onto a plate lined with paper towels to absorb the oil.
Clean the skillet and put the meatballs back in. Pour the marinara sauce on top. Cover the skillet with a tight lid and cook for an additional 10-15 minutes on low heat.
Garnish with grated Parmesan cheese or chopped parsley.