In a large pot, heat a bit of oil over medium heat.
Add the massaman curry paste and sauté for 1-2 minutes until fragrant.
Add the cubed meat and cook until browned on all sides.
Pour in the coconut milk and stir well to combine.
Add the diced potatoes and sliced onion.
Stir in fish sauce and brown sugar.
Add water to cover the ingredients, then bring to a boil.
Reduce heat and let simmer for 40-50 minutes, or until meat is tender.
Stir in roasted peanuts and cook for an additional 5 minutes.
Serve hot, garnished with fresh cilantro.