Combine panko breadcrumbs, milk, onion, salt, five spice powder, and black pepper in a large mixing bowl until even. Add chicken,. Use your hands to gently combine all of the ingredients. Form into meatballs that are about 1.5-inches in diameter (these should be fairly small; if the mixture sticks to your hands, wet your hands with a small amount of oil or water).
Whisk together all of the sauce ingredients until smooth. (You may need to whisk a few extra times to get rid of any dried cornstarch lumps.) Set aside.
Set a paper towel-lined plate near the stove.
Heat a large saute pan or skillet over medium heat. Add cooking oil. When cooking oil is hot, add peanuts and saute just until fragrant and toasted, 1 to 2 minutes (keep a close eye on them - they can burn quickly if they get too hot).
Use a slotted spoon to transfer the peanuts to the paper towel-lined plate. Leave the oil in the pan.
Add meatballs to remaining oil in the pan, Let meatballs cook on one side until golden brown, then gently turn meatballs until evenly golden brown on all sides, 6 to 8 minutes total. (Note: If the pan starts to look dry or the meatballs start to stick at any point during cooking, add some extra oil.
Add diced red bell peppers to meatballs. Pour sauce over top. Simmer sauce until it thickens slightly, turning meatballs to coat in sauce.
Remove pan from heat and top meatballs with peanuts and green onions.
Serve meatballs over rice.