Preheat your oven to 400°F (200°C). Season both sides of the filet mignon steaks with salt and pepper.

In a bowl, combine the toasted macadamia nuts and chopped tarragon. Press this mixture onto the top of each seasoned steak to create a crust.

In a large oven-safe skillet, heat the olive oil over medium-high heat. Add the steaks crust-side down and sear for 3-4 minutes until golden brown. Flip the steaks and sear for an additional 2 minutes on the other side.

Transfer the skillet to the preheated oven and cook the steaks to your desired doneness, about 5-7 minutes for medium-rare. Remove from the oven and let rest for 5 minutes.

In the same skillet, remove excess fat and add the sliced onion. Sauté over medium heat until the onions are charred and caramelized, about 8-10 minutes.

Stir in the beef broth, Worcestershire sauce, and balsamic vinegar. Scrape the bottom of the skillet to deglaze and mix well. Allow the gravy to simmer for 5 minutes until slightly thickened.

Remove the skillet from heat and stir in the butter until melted and the gravy is velvety smooth. Season with salt and pepper to taste.

Slice the filet mignon and spoon the charred onion gravy over the top. Garnish with fresh tarragon sprigs. Serve immediately.