In a large pot or Dutch oven, heat the vegetable oil over medium-high heat. Add the beef cheeks and sear on all sides until browned. Remove the beef and set aside.
In the same pot, add the chopped onion and garlic. Sauté until the onion is translucent, about 5 minutes.
Stir in the Massaman curry paste and cook for another 2 minutes until fragrant.
Return the beef cheeks to the pot. Pour in the coconut milk and beef broth, stirring to combine.
Add the diced potatoes, sliced carrots, fish sauce, brown sugar, and lime juice. Bring to a simmer.
Cover the pot and reduce the heat to low. Let it cook for 2 to 2.5 hours, or until the beef is tender and easily shredded.
Stir in the chopped peanuts just before serving. Garnish with fresh cilantro.