Preheat oven to 350 degrees.
Melt 2 Tbsp of butter in a large skillet over medium low heat. Add in the onion, brown sugar and salt, stir and cook until softened, 5 to 10 minutes.
Meanwhile, place the bottom of the rolls in a baking dish. Lay slices of cheese over the roll bottom. Next, lay 1 slice of roast beef on each roll, folding it over to fit on the roll.
Next, spoon about 2 Tbsp. of sauerkraut and 2 Tbsp. of the onions over the meat. Add another layer of cheese. Place the bun back on top.
Melt remaining 2 Tbsp. butter in the same pan in which the onions were cooked. Brush or drizzle over the tops of the rolls. Sprinkle with oregano.
Bake for 10-15 minutes or until cheese is melted. Serve!