Prepare the Fries: Peel the russet potatoes and cut them into thin fries. Rinse the fries in cold water to remove excess starch, then pat them dry with a paper towel. In a deep pot, heat vegetable oil to 325°F (160°C). Fry the potatoes in batches for about 5-6 minutes until soft but not browned. Remove and drain on paper towels. Increase the oil temperature to 375°F (190°C). Fry the potatoes again in batches for about 3-4 minutes until golden brown and crispy. Remove and drain, then season with salt.

Cook the Sirloin Steaks: Season the sirloin steaks generously with salt and black pepper on both sides. In a large skillet, heat 2 tablespoons of olive oil over medium-high heat. Add the steaks and sear for 4-5 minutes on each side for medium-rare, adjusting time for desired doneness. Add 1 tablespoon of butter, smashed garlic, and thyme (if using) during the last minute of cooking, spooning the melted butter over the steaks. Remove the steaks from the skillet and let them rest for at least 5 minutes before slicing.

Make the Mushroom Peppercorn Sauce: In the same skillet, add 1 tablespoon of olive oil and sauté the shallots and mushrooms over medium heat until softened, about 4-5 minutes. Add crushed peppercorns and cook for an additional minute. Pour in the beef broth, scraping up any brown bits from the pan, and let it reduce for about 3-4 minutes. Stir in the heavy cream and Dijon mustard, bringing the sauce to a gentle simmer. Cook for another 3-5 minutes until thickened. Season with salt to taste.

Serve: Slice the rested sirloin steaks against the grain and serve alongside the double-fried fries. Drizzle the rich mushroom peppercorn sauce over the steak and garnish with fresh parsley.