Build a fire (or light a grill) and let it burn down until the coals are piping hot. Place a grate in a secure position 3-4 inches above the fire.
Place a sheet of foil on a flat surface. Spread the butter across the foil. Pile the spinach in the center of the foil, then add the mozzarella, Gruyère, and chopped onion. Mix the cheeses, onion, and spinach together, then add the Greek yogurt and quickly fold into the spinach and cheese mixture.
Wrap the foil tightly around the spinach to form a pouch. Use a second sheet of foil to create a second layer, to prevent leaks when flipping the pouch. Place the spinach-filled packet onto the grate over the fire. Cook for 10-15 minutes, flipping once, until the spinach has reduced to half its pre-cooked size and the onions have softened.
While the spinach cooks, skewer each hot dog on a long roasting stick. Use a knife to slice into the hot dogs at a forty-five-degree angle. Spin the hot dogs as you cut them. Gently pull the hot dog so that it spreads apart down the skewer.
Roast the hot dogs on their skewers over the hottest part of the fire, rotating frequently to cook evenly. Remove the spinach from the fire when the hot dogs are ready.
Spread the cheesy spinach mixture across the tops of the spiralized hot dogs, making sure to get it into the spirals of the dog. Serve on toasted buns and enjoy!