In a large stockpot over medium heat, add the oil and onions and cook for 2-3 minutes or until onions are slightly translucent and soft. Add the chicken and curry paste, stir together, and cook for 5-7 minutes. Add potatoes and stir.
Pour water and coconut cream into the pot then bring to boil and lower heat to simmer for about 20-30 minutes, until the chicken and potatoes are cooked through. Add the fish sauce and brown sugar and stir to combine.
Remove from heat and serve immediately alone or over rice.