Start by cooking the bacon. Cook for about 6 minutes then turn and cook for an additional 6-8 minutes.
Take the loaf of bread and cut it in half. Spread with butter and place face down on a cookie sheet. Put into a preheated 350-degree oven and let it toast up for about 5-8 minutes.
Once the bacon is done remove it from the pan and put on a plate with paper towels to allow it to drain. Remove all the grease in the pan from the bacon reserving about 1-2 tsp.
Add 1 Tbs of butter to that then add the sliced mushrooms and onions. Let those cook down for about 5 minutes. Then add the roast beef.
While those cook flip the bread over and spread the cheese equally on each side. Put back into the oven and let the cheese melt, around 5 minutes.
Now make the sauce which is just mixing the blue cheese, mayonnaise and horseradish sauce together.
Crumble the bacon and add to the cooked roast beef.
Take the bread out of the oven then pile it high onto one side of the bread then top off with the other. Cut into 6 pieces then serve. Let others add lettuce, tomatoes, and the blue cheese mixture as they like. Enjoy!