In a medium saucepan, combine the sliced onions and olive oil. Add the thyme, salt, and black pepper. Cook over low heat for about 45 minutes, stirring occasionally, until the onions are caramelized and tender. Remove from heat and set aside.

Preheat the oven to 400°F (200°C). Season the flat iron steak with salt and black pepper on both sides. In a mixing bowl, combine the breadcrumbs, chopped rosemary, and a pinch of salt and pepper. Brush the Dijon mustard evenly over the steak, then press the breadcrumb mixture onto both sides of the steak to create a crust.

In an oven-safe skillet, heat a tablespoon of olive oil over medium-high heat. Once hot, add the crusted steak and sear for about 3-4 minutes on each side until golden brown. Transfer the skillet to the preheated oven and roast for an additional 8-10 minutes for medium-rare, or until desired doneness. Remove from the oven and let the steak rest for 5 minutes.

In a small bowl, mix the softened butter with cracked pink peppercorns and truffle salt until well combined. Set aside.

Slice the rested flat iron steak against the grain. Plate the steak and top with the confit onions. Add a dollop of the truffle-salt butter on top of the steak. Garnish with fresh parsley.