Place chicken in a large bowl.
Heat large saute panh over medium heat. Once hot, add half the olive oil and butter. Add the onion and 1-2 pinches of salt and cook until the onion is golden brown, approximately 6-7 minutes. (Stir often!)
Cool slightly, then add to the bowl with the ground chicken along with the panko, egg, milk or water, 1 teaspoon salt, and black pepper to taste. Stir with a fork to combine.
Scoop up approximately 2 tablespoons of the meatball mixture and roll back and forth with your wet hands to form a meatball. (Or use an ice cream or cookie scoop - that works great too.) Arrange on a plate.
Add the remaining butter and olive oil to the saute pan over medium heat. Arrange the meatballs in one layer. (They will seem soft but don't worry - as soon as they cook for a couple of minutes, they begin to hold together better.) Let them brown on the buttom and then carefully roll them around the pan until they are evenly brown on each side. Remove the meatballs with a slotted spoon and drain on paper towels.