Start by peeling and slicing the 2 small onions into thin rings. This will allow them to caramelize beautifully, adding a sweet flavor to your sandwich.
In a large pan or skillet, melt 2 tablespoons butter over medium-high heat. The butter should foam slightly but not brown; this is a sign it's ready for the onions.
Add the onion slices to the pan along with ¼ teaspoon salt. Sauté the onions until they soften, about 5 to 7 minutes. You'll notice them turning translucent and fragrant.
Once the onions are soft, increase the heat to high. Keep stirring them as they cook, watching for them to turn a golden amber color. This caramelization adds a rich flavor.
Remove the onions from the pan and set them aside. This will keep them warm while you prepare the rest of the sandwich.
In the same skillet, pour in 2 cups broth. There's no need to clean the pan; the leftover butter and bits will enhance the broth's flavor. Bring the broth to a simmer.
Carefully add the 12 slices turkey to the simmering broth, heating gently for about 2 to 3 minutes until it's warmed through.
Meanwhile, on 4 pieces of 8 slices bread, place a slice of 4 slices cheese on each. Top with the caramelized onions for added sweetness.
Once the turkey is heated, divide it among the 4 pieces of bread. Sprinkle with ½ teaspoon black pepper to add a slight kick.
Close the sandwiches and serve them hot with 1½ cups hot gravy for dipping. This elevates the sandwich, making each bite even more satisfying.