Cook filet or sirloin to medium rare and let rest for 15 minutes. (I drizzled a little olive oil into hot skillet and seared both sides, salt and peppering both sides, and cooked to medium rare.) Let rest on cutting board for 15 minutes. Slice in 1/8 to 1/4 inch slices. In a large skillet, melt butter and olive oil. Add sliced onions and garlic. Mix and stir for a few minutes to soften. Add white and Crimini mushrooms. Cook for about 10 minutes on medium high heat, stirring occasionally. Add balsamic reduction, parsley, salt and pepper and cook for another 5 minutes. Slice baguettes in ¼ inch slices and lay on baking sheet. Drizzle lightly with olive oil. Sprinkle grated gruyere (or hard cheese) on each baguette slice and bake for about 8-10 minutes at 350 degrees or until bread is golden brown and cheese is melted. Assemble. Lay a slice of beef over top of bread with melted cheese. Place a heaping teaspoonful of mushroom-onion mixture over top of beef. Garnish with tiny amount of parsley.