In a sauté pan, add olive oil and onion. Cook gently on medium-high heat until onions are translucent.
Add mushrooms, season with salt and pepper and continue cooking till mushrooms wilt a little.
Remove from pan and let cool.
To assemble the sandwich, slice open baguettes, leaving one edge intact. Spread mustard on one side of each. Then lay on sliced beef, followed by the onion/mushroom combination, sliced tomatoes and cheese, if using.
Place under a broiler (or, if you're preparing only one sandwich, use a toaster oven) until the cheese is melted. Then fold together, and enjoy.