Place the banana peppers, shallots, garlic, salt, and chilli powder with vegetable oil in a blender, and pulse until the mixture is a paste.
Place the beef tenderloin cubes in a skillet over medium heat with the pepper mixture, galangal, lemon grass, lime leaves, and turmeric. Cook the beef with the seasonings,stirring occasionally, until the meat is no longer pink inside and the meat juices have mostly evaporated, about 15 minutes.
Pour the coconut milk into the skillet, bring to a boil, reduce the heat, and simmer for about 15 minutes. Remove the pieces of beef to a bowl, and let the coconut milk continue to simmer until thickened, about 30 minutes. Return the beef to the coconut sauce, reduce the heat to low, and barely simmer the beef and sauce until the sauce turns brown and the oil has separated from the coconut milk, about 2 1/2 hours. Stir frequently as the sauce thickens.