Heat oil in a large wok over medium heat. Add massaman curry paste and stir-fry until fragrant.
Add water and bring to a boil. Once the curry paste oil rises to the top, pour in the coconut milk and let it come to a full boil.
Add chicken legs, palm sugar, fish sauce, salt, and tamarind paste. Simmer 5–10 minutes, flipping halfway, until the chicken surface is opaque.
Add potatoes, cover with a lid, and simmer 20 minutes or until the chicken is cooked and potatoes are fork-tender.
Add chopped onion and simmer a few more minutes.
Garnish with roasted peanuts and serve with jasmine rice. Enjoy!