1. Preheat the oven to 220 °C (428 °F). Finely chop the onion. Wipe or peel the mushrooms and slice them. Heat a drizzle of olive oil in a skillet over medium-high heat and sauté the onion and mushrooms for 4 to 5 minutes, stirring regularly.
2. Add the sliced beef, season with salt and pepper. Mix well and cook for an additional 1 to 2 minutes.
3. Split the baguettes in half without completely separating them. Place them open-faced, crumb side up, on a baking sheet lined with parchment paper. Spread the contents of the skillet onto the baguettes, top with raclette cheese, and bake for 5 to 7 minutes until the cheese melts and slightly browns.
4. Meanwhile, in a salad bowl, prepare a vinaigrette (for example, per person, 1/2 tablespoon of olive oil, 1/2 teaspoon of mustard, 1/2 teaspoon of black balsamic vinegar, salt, and pepper, and mix well).
Just before serving, add the salad to the vinaigrette and toss. Serve the open-faced sandwich with the salad on the side.