Pre-Heat Oven to 350 Degrees
In a skillet, melt butter over medium-high heat. Add mushrooms, onions and 1/4 Cup of Guinness and sauté until liquid is absorbed.
Prepare glaze by combining all ingredients in a medium saucepan.
Bring mixture to a boil, then lower heat slightly and cook until thickened.
Assemble sandwiches by slicing rolls in half, then filling with roast beef, sautéed mushrooms & onions, and swiss cheese.
Pour glaze evenly over sandwiches.
Bake for 30 minutes.