Heat oil in sauté pan over medium high heat. Add garlic, onions and mushrooms and cook for 3 minutes, stirring constantly. Remove from pan and reserve. Wipe pan to clean.
Butter outside surface of each slice of bread.
Layer beef onto unbuttered side of bread, top with barbeque sauce, Fontina, onion mixture and another slice of bread (buttered side out). Repeat for second sandwich.
Heat sauté pan over medium low. Cook sandwich until cheese is fully melted, approximately 4-5 minutes per side. Serve with warm au jus for dipping.
