Heat the vegetable oil in a large pot over medium heat. Add the massaman curry paste, chopped onion, and minced garlic.
Add the ginger and chicken pieces to the pot and sauté until the chicken starts to brown, about 5 minutes.
Add the potatoes, carrot, coconut milk, fish sauce, peanut butter, tamarind paste, and brown sugar to the pot.
Stir in the lime juice and chopped peanuts and cook for an additional 5 minutes. Adjust seasonings and add salt if desired.