Heat Extra Melt Cheese Sauce according to package directions.
Combine warm cheese sauce, blue cheese crumbles and hot water in medium stockpot. Heat to 165°F, stirring occasionally. (Some of the blue cheese will not fully melt.) Hold warm until service.
To build hoagie, place 4 ounces meat into opened bun. Top with 2 ounces mushrooms and 1 1/2 ounces onions. Drizzle with 2 ounces warm blue cheese sauce.
Serve sandwich warm.