Preheat your oven to 400 degrees F. Prepare a sheet pan by covering it with parchment paper or aluminum foil sprayed with nonstick spray.
Combine the ground chicken, breadcrumbs, egg, milk, sesame oil, soy sauce, grated ginger, garlic powder, salt, and pepper in a large mixing bowl.
Roll the meat mixture into 24 to 26 meatballs and place them on the sheet pan, leaving space between each one.
Place the pan in the oven and bake for about 10 minutes. When the time is up, remove the pan from the oven and set aside.
While the meatballs cook, set a large dutch oven or high sided skillet over medium heat. Add olive oil.
When the oil is hot, stir fry the onions and peppers. Cook for 3 to 5 minutes, then add the pineapple and cook 1 more minute.
Remove the vegetables from the skillet and set them aside. Turn the stove to medium-low heat and add the sauce ingredients besides the corn starch and water.
Bring the sauce to a boil. Whisk together the corn starch and water in a small bowl. Then add the mixture to the sauce.
Simmer the sauce until it starts to thicken, about 5 minutes. Whisk every so often so the bottom doesn't burn.
Once the sauce is thick enough to coat the back of a spoon, add in the stir fried vegetables and stir to coat them in the sauce.
Add the chicken meatballs to skillet and gently stir them to coat in the sauce as well. Be careful not to break up the meatballs.
Remove the skillet from the heat and serve sweet and sour chicken meatballs over white or brown rice.