Preheat oven to 350 degrees.
Add oil and brown sugar to a large frypan and bring to medium heat for about 5 minutes. Continue to stir together until the sugar has melted.
Add the chicken pieces in batches to frypan and brown on both sides until the pieces are a nice golden brown. Transfer chicken pieces as they are ready to a large casserole dish and set aside.
Keep the brown bits and any remaining oil mixture in the pan, and toss in any remaining marinade. Add in onion slices and stir to coat with marinade mixture.
Continue cooking onions on medium heat by stirring until they are soft and transparent. About 10 to 15 minutes.
Add in diced tomatoes and stir to combine with onions. Pour tomato mixture over chicken and cover with foil.
Bake until bubbling for 60 minutes. Remove foil and bake for an additional 15 minutes.
Remove from oven and let sit for about 10 minutes and serve over rice.