Begin by cutting the beef chuck into even 2-inch pieces to ensure uniform cooking. Set aside.

In a food processor, combine the shallots, garlic, ginger, galangal, serrano and Fresno chiles, salt, Korean red pepper flakes, coriander, turmeric, cardamom, and nutmeg. Blend until a smooth, aromatic paste forms, pausing occasionally to scrape down the sides for even consistency.

Heat the vegetable oil in a large pan or Dutch oven over medium heat. Add the spice paste and sauté, stirring continuously, until it becomes fragrant and begins to darken slightly, indicating the rawness of the spices has cooked out.

Add the beef pieces to the pan and stir to coat them thoroughly with the paste. Pour in the coconut milk, then stir in the tamarind paste and brown sugar. Fill the empty coconut milk can with water and add it to the pan as well, ensuring the beef is mostly submerged.

Using the back of a knife, bruise the lemongrass stalk to release its oils and aroma, then cut it into manageable segments and drop them into the mixture. Bring the curry to a gentle boil over medium-high heat.

Once bubbling, reduce the heat to medium and simmer uncovered. Allow the rendang to cook low and slow for about 4 hours, stirring occasionally. As the sauce thickens and reduces, stir more frequently to prevent sticking or scorching. If the liquid reduces too quickly before the beef is fully tender, lower the heat or add a bit more water as needed.

Continue cooking until the beef is fork-tender and the sauce has darkened, clinging thickly to the meat in a rich, caramelized coating. Before serving, remove and discard the lemongrass pieces.

For the most developed flavor, allow the rendang to cool completely and refrigerate overnight. Reheat gently before serving for best results.