Heat the oil over medium high heat in a large Dutch oven. When it is shimmering, brown the beef on all sides, which will take a good 20 minutes or so. When it is brown, remove the beef to a large bowl.
Drain the excess fat off of the pot and add the coconut fat to the pot. As soon as it melts and starts to separate, add the curry paste. Mix it together and fry--scrape the bottom of the pot to deglaze. You want the mixture to separate, where you will see the curry paste fat separating from the rest of the mixture. When this happens, add the rest of the coconut milk with 2 tablespoons each of fish sauce and palm sugar. Mix. Return the meat to the pot, coating it on all sides. Cover the pot with a heavy, secure lid and place in the oven.
2 hours before serving the curry, remove it from the oven and add the potatoes. Turn the heat to 300 F if it was not already and return the lid to the pot. Return it to the oven for an hour.
1 hour before serving, sprinkle the onions and peppers all over the curry. Replace the lid, return it to the oven, and increase the heat to 325 F. If you are working on a tight schedule, check the curry after about 45 minutes to make sure the potatoes and onions are getting tender enough--if not, increase the heat to 350 F. If you are laid back, just let time do the work for you.