Prepare the Shoestring Fries: Peel the russet potatoes and slice them into thin shoestring strips using a mandoline or sharp knife. Rinse the potato strips in cold water to remove excess starch, then pat them dry with a clean towel.

In a deep pot, heat vegetable oil to 350°F (175°C). Carefully add the potato strips in batches, frying until golden brown and crispy, about 3-4 minutes per batch. Remove with a slotted spoon and drain on paper towels. Sprinkle with truffle salt while still hot.

Cook the Filet Mignon: Season the filet mignon steaks generously with salt and black pepper on both sides. In a large skillet, heat the olive oil over medium-high heat. Once hot, add the steaks and sear for about 4-5 minutes on each side for medium-rare, depending on thickness.

During the last minute of cooking, add the butter and rosemary to the skillet. Tilt the pan and use a spoon to baste the steaks with the melted butter and rosemary oil. Remove the steaks from the pan and let them rest for at least 5 minutes.

Make the Savory Mushroom Jus Sauce: In the same skillet used for the steaks, add the minced shallot and garlic, cooking for about 1-2 minutes until fragrant.

Add the chopped mushrooms and thyme, sautéing until the mushrooms have released their moisture and are golden brown, about 5-7 minutes.

Pour in the beef broth, soy sauce, and balsamic vinegar, stirring to combine. Bring to a simmer and let it reduce slightly, about 5-10 minutes. Remove from heat and stir in the butter. Season with salt and pepper to taste.

Serve: Slice the filet mignon and arrange on plates. Serve alongside the crispy shoestring fries and drizzle the savory mushroom jus sauce over the steak. Enjoy your delicious meal!