Slice the onions into thin half round slices. Try to slice them as evenly as possible. Place the sliced onions, ½ cup water, and olive oil in a non-stick pot over medium high heat. Keep your eye on the onions, stirring frequently, and adding more water to the pot as necessary. You can keep the onions over high heat until they are translucent and soft. Then reduce the heat to medium. Continue to add water to the pan to prevent the onions from sticking. Stir frequently. It will take about 30 minutes for the onions to become golden brown.

Heat your oven to 350 degrees.

Without separating the dinner rolls, place them all on a baking sheet. Then use a serrated knife to slice the tops off all of the rolls. Remove the top of the rolls. Spread both halves of the rolls with mustard. Then stack the spinach, half the cheese, the roast beef, the caramelized onions, and the remaining cheese on top of the bottom halves of the rolls. Then stack the tops of the rolls on top, gently pressing them into place.

Brush the top of the rolls with melted butter.

Bake for 20 minutes.

Slice the rolls and serve!