Separate the pineapple fruit from its juice. Save the juice. Slice the pineapple in chunks and store in the fridge until it's time to cook.
Place the chicken pieces in a ziploc bag or large container with cover. Pour the pineapple juice into the bag. Massage gently then store in the fridge to marinate for 2-3 hours. (30 minutes would be fine if you're in a hurry)
Heat up oil in a pan. Sauté chopped onions and garlic.
Add the chicken pieces into the pan to brown both sides. 3-5 minutes on each side. Do not throw the marinade away.
Add the marinade juice and milk into pan. Stir gently and let it boil.
Stir in cubed potatoes and pineapple chunks into the pan. Stir, cover, then simmer for 35-40 minutes.
Add sweet chili sauce(optional) and soy sauce. Stir gently then add the sweet peppers. Stir everything together and let it cook for 5 more minutes.