Prep all ingredients per instructions above.
Cook the rice according to the directions on the package.
Add half of the coconut milk to a skillet, then cook it over medium heat until it breaks or separates (the oil separates). Next, add the massaman curry paste and stir-fry until fragrant. Add the chicken to the skillet and the rest of the coconut milk. Pour in enough water to cover. Next, add fish sauce, tamarind juice, and sugar. Bring to a boil, then simmer for 20 minutes.
Add the potatoes and the onions and simmer for 20 more minutes. If the vegetables and the chicken cook sooner (it's done when fork-tender), take off the heat momentarily.
Finally, add the peanuts and adjust the seasonings — more fish sauce if it needs salt, more tamarind juice to get the right acidity, and more sugar to make it sweeter.
Warm the naan in a nonstick pan over medium heat on both sides.
Serve curry with Jasmine rice, naan, and garnish with cilantro. Enjoy with coconut water.