Heat the olive oil in a frypan (or use the sear function on your slow cooker if you have one), and brown both sides of the steak, then add it to the slow cooker (it will not be cooked through at this point).
1 tbsp olive oil, 1 kg beef chuck steak
Add the onions to the frypan and cook until they start to soften.
1 medium brown onion
Add massaman paste and continue to fry until it becomes fragrant (about 1 minute).
114 g massaman curry paste
Add peanut butter, coconut milk, and beef stock, and stir to combine.
2 teaspoons peanut butter, 400 ml coconut milk, ½ cup beef stock
Pour the coconut milk mixture over the beef in the slow cooker.
Nestle the star anise (see my notes) and potatoes into the sauce around the beef and put the lid on.
1 star anise, 400 g baby potatoes (whole)
Cook for 8 hours on low or until the beef is tender and cooked through.
To serve, discard the star anise, skim off any excess oil, remove the beef and shred (discarding any visible fat).
Optional step: for a thicker sauce, pour the sauce into a small saucepan and simmer on the stovetop.