Heat the vegetable oil in a large skillet over medium-high heat.
Add the beef cubes and brown them on all sides, about 5-7 minutes. Transfer the browned beef to the slow cooker.
In the same skillet, add the sliced onion and sauté for 3-4 minutes until softened.
Add the Massaman curry paste to the onions and cook for 1-2 minutes, stirring constantly to release the aromas.
Pour in half of the coconut milk and stir well to combine with the curry paste and onions. Cook for 2 minutes.
Transfer the coconut milk mixture to the slow cooker with the beef.
Add the potatoes, roasted peanuts, fish sauce, brown sugar, tamarind paste, cinnamon stick, cardamom pods, bay leaves, and beef broth to the slow cooker.
Pour the remaining coconut milk over the ingredients in the slow cooker.
Stir gently to combine all ingredients.
Cover and cook on low for 6 hours, or until the beef is tender and the potatoes are cooked through.
Remove the cinnamon stick, cardamom pods, and bay leaves before serving.
Serve the beef Massaman curry hot over cooked jasmine rice and garnish with fresh cilantro leaves.