Cut the meat into long chunks about 1 1/2-inch wide. Visually inspect the chunks: if they appear to have less than 20-25% fat, mix them with strips of beef fat (describe "braiding" technique). Season well with salt and pepper.
In a meat grinder, grind together the boneless meats through a large grinding blade (with holes of 3/8"). Grind again. Loosely shape into 12 hamburgers.
If cooking indoors, heat a large, heavy skillet over high heat. Add 2 ounces of beef suet and let it melt. Add six burgers to the skillet if they fit without crowding. Cook about 4 minutes on each side for rare burgers, or longer if desired. Repeat in the same beef suet with the remaining six burgers.