Cook the ground beef in a large skillet over medium-high heat, stirring to crumble, until browned. Pour off excess fat, and mix in the taco seasoning according to package directions.
Spray the inside of a pie iron with cooking spray, and place a corn tortilla on one side. Place a scoop of ground beef on top of the tortilla, sprinkle with Monterey Jack cheese, and chopped onion. Finally, place another tortilla on top, and close the pie iron.
Cook the taco over the moderate coals of a campfire until the tortillas have crisped and browned, and the taco is hot in the center. Serve with lettuce, tomato, salsa, and sour cream. Repeat with remaining ingredients.