Preheat oven to 400 degrees F. Place bread on a work surface and using a 3-inch cookie cutter, cut a round crouton from the center of each piece of bread. Brush both sides of the croutons with olive oil and season with salt and pepper, to taste. Place the croutons on a baking sheet and bake for 5 to 7 minutes or until lightly golden brown.
Tournedos and Sauce:
Heat olive oil and butter in a large skillet over high heat. Season the fillets with salt and press the coarsely cracked black pepper onto 1 side of each fillet. Place the fillets pepper-side down into the skillet and sear until golden brown, turn the fillets over and continue cooking for 4 to 5 minutes for medium-rare doneness. Remove the steaks to a plate. Remove all but 3 tablespoons of the fat in the pan and return to high heat. Add the mushrooms and cook until golden brown and their liquid has evaporated. Add the shallots and cook until soft. Add the red wine and reduce by half. Add the broth and cook until reduced by half. Whisk in the mustard and butter and cook for 30 seconds. Season with salt and pepper, to taste. Place each fillet on top of a crouton and spoon some of the sauce over.
Onion and Potato Gratin:
Heat oil in a large saute pan over medium heat. Add the onions and cook slowly until caramelized. Add the garlic and balsamic vinegar and cook for 2 minutes. Preheat oven to 375 degrees F. Butter a 9-inch baking pan a place a layer of potatoes on the bottom and season with salt and pepper. Spoon 1/5 of the onion mixture over the potatoes and top with 1/4 cup of the heavy cream and a tablespoon of parsley. Repeat to make 5 layers. Sprinkle the cheese over the top of the potatoes. Bake for 50 to 60 minutes, or until the potatoes are soft and the cream has been absorbed.