Heat Vegetable Oil In A Large Skillet Over Medium Heat. Stir In Chicken, And Cook Until The Pieces Are No Longer Pink In The Center, About 5 Minutes. Remove Chicken, Then Stir In The Red Curry Paste And Sugar; Cook And Stir For 1 Minute. Add The Bell Peppers And Onion To The Pan, And Cook For 2 Minutes. Pour In The Coconut Milk, Return The Chicken To The Pan, And Bring To A Simmer Over Medium-High Heat. Cook Until The Onions And Peppers Are Tender, About 5 Minutes. Stir In The Fish Sauce And Serve.