This recipe looks like a significant commitment but, except for assembly and plating, it can all be done ahead of time. These little bites are exceptionally delicious and celebratory. When you serve them to rave reviews, it will all be worthwhile.
Toss onions with salt, balsamic vinegar, and olive oil. Cover and roast at 350°F for 1 hour.
Uncover and roast for another hour, stirring a couple of times for even cooking. Use immediately or place onions in a jar and refrigerate up to one week.
Rub beef filet with mushroom powder, coffee, salt, and pepper. Refrigerate overnight if possible or at least 2 hours.
Take the beef out of the refrigerator and allow it to come to room temperature.
Heat a cast-iron pan - add 1 Tbsp. olive oil and when it starts to shimmer, add beef.
Cook for approximately 3 minutes turning to cook evenly on all sides for approximately another 3 minutes. Beef should be rare to medium-rare (130 F - 135 F), depending on your preference.
Let beef rest for at least 15 minutes and then slice very thinly.
Top each baguette piece with beef, balsamic onions, and cheese.
Drizzle with olive oil and serve.