Pre-heat the oven to 350 degrees Fahrenheit.
In a large mixing bowl, combine the ground beef and the chorizo.
In a blender, add the onions, black pepper, salt, cumin, and garlic and mix at medium speed adding water. Pour the blender mixture into the beef mixture, mix well, and form into burger patties by hand.
Cook burger on a skillet, grill pan, or grill to desired temperature. Butter the buns and heat in the oven for 5 minutes.