In a large (about 6 quart) pot brown the chicken in 2 tbsp oil, add the onion when chicken is almost cooked to help soften them.
If you are using whole fat coconut milk, be sure to shake the can! Add the 2 cans of coconut milk to the pot with the creamy peanut butter, mild curry paste, brown sugar and fish sauce. Stir until combined.
Once hot, add the frozen peas.Continue cook until bubbly.
If the sauce is not thick enough, mix the corn starch with the water and add to the pot while the sauce is bubbling to activate the corn starch as a thickener.
Serve over cooked rice and garnish with peanuts if desired.