Melt 1 tablespoon of butter in a large skillet over medium-low heat
Add the onion, brown sugar, and salt to the skillet. Stir and cook until softened about, 5 to 10 minutes. Remove to onions to a bowl and set them aside for now.
Cut the Hawaiian rolls in half so you have tops and bottoms. (It's easier if you don't pull them apart.) Place the bottom of the rolls in a baking dish.
Lay 6 slices of cheese over the roll bottoms so they're completely covered.
Place 1 slice of roast beef on each roll, you will have to bunch it up to make it fit. Spoon about 1 teaspoon of the onions onto each roast beef slice.
Lay the remaining 6 slices of cheese over the top of the onions. Place the tops of the roll on top of the cheese.
Melt the remaining 1 tablespoon of butter and then brush over the tops of the rolls. Sprinkle the rolls with oregano if desired.
Bake for 10-15 minutes or until cheese is melty and the tops of the roll have browned.
Remove from the oven and cut the sliders apart before serving warm.
Notes
If you're having trouble finding horseradish cheese (we get ours at the deli counter), spread some creamy horseradish on the cut side of the bottom and tops of the rolls and use regular white cheddar cheese.