Heat the vegetable oil in a large heavy-bottomed pot or Dutch oven over medium-high heat. Add the beef cubes in a single layer and sear until deeply browned on all sides, about 5 to 7 minutes. Remove the beef and set aside on a plate.

Reduce the heat to medium and add 1/2 cup of the coconut milk to the pot. Cook for 2 to 3 minutes, stirring continuously, until the milk reduces slightly and the natural coconut oils begin to separate from the cream.

Stir the Massaman curry paste into the reduced coconut milk. Cook until highly fragrant, about 1 to 2 minutes, allowing the spices to toast and release their aromatic flavors.

Return the browned beef to the pot along with the remaining coconut milk, beef broth, star anise, and cinnamon stick. Bring the mixture to a gentle boil, then reduce the heat to low, cover, and simmer for 1 hour to begin tenderizing the meat.

After 1 hour of simmering, add the potatoes, chopped onion, and roasted peanuts to the pot. Stir gently to combine, cover again, and continue to simmer for another 30 to 40 minutes, or until the beef and potatoes are completely fork-tender.

Stir in the brown sugar, fish sauce, and tamarind paste. Simmer uncovered for the final 5 minutes to thicken the sauce slightly and ensure the sweet, savory, and tangy flavors are perfectly balanced.

Remove the pot from the heat and carefully fish out and discard the star anise and cinnamon stick. Serve hot in bowls over freshly steamed jasmine rice, garnished with a few extra roasted peanuts if desired.