On a large work surface, slice the rolls in half through the middle. If they are attached as dinner rolls often are, you can leave them together in groups of six, and slice horizontally through each large mass of rolls.
On the bottom half of the rolls, layer on the ham. Add the cheese in another layer. On the top half of bread, spread the mustard and sprinkle the onions. Place the two halves together and wrap tightly with aluminum foil. At this point the rolls can be frozen, for up to 1 month. Alternatively, you can refrigerate the sandwiches up to a day in advance.
Thaw the rolls completely in the fridge prior to baking, if frozen. Preheat the oven to 350 degrees. Place the wrapped foil package in the oven and bake for about 25 minutes. At this point, slice through the rolls and separate. If not heated completely through, bake for another 10 minutes or until ham is hot, cheese is melted, and bread is crisp. Serve immediately.
Notes
Unbaked sliders can be assembled a day in advance or frozen for up to 1 monthÂ