For Onions:
Melt butter in a large pot over medium heat. Thinly slice onion and add to the large pot. Season with salt and sugar.
Reduce heat to medium low and cook until onions begin to caramelize. Stir frequently to prevent burning. This will take 15-20 minutes. Remove from heat and allow to cool on a plate.
For Curry:
In a medium bowl, mix together chicken, salt, pepper, garlic powder, and oil.
Heat a large rimmed skillet over medium heat. Add the chicken and lightly brown on all sides. Add the potatoes and mushrooms. Stir to combine. All mixture to cook for about 6 minutes.
Add the fish sauce, caramelized onions, and sugar. Stir to evenly coat the chicken and veggies. Cook for an additional 2 minutes.
Stir in the coconut milk and red curry paste. The sauce should become a light red/orange color and evenly mixed. Simmer for 5 minutes.
Serve with hot cooked rice.
