Preheat the oven to 400°F (200°C). Season the flat iron steak generously with salt, pepper, and cracked black pepper. Rub with Dijon mustard to coat evenly.

In a skillet, heat 1 tablespoon of olive oil over medium heat. Add the pearl onions and sauté for about 5 minutes or until they begin to caramelize. Remove from heat and set aside.

In the same skillet, add the remaining tablespoon of olive oil and increase the heat to high. Sear the steak for about 3-4 minutes on each side, until browned.

Once the steak is seared, remove it from the skillet. Add the grapefruit zest and segments to the skillet, stirring to combine. Place the steak back in the skillet, spoon some of the grapefruit mixture over the top, and top with the sautéed pearl onions.

Transfer the skillet to the preheated oven and roast for about 10-15 minutes for medium-rare, or until your desired doneness is achieved. Use a meat thermometer to check the internal temperature (130°F for medium-rare).

While the steak is roasting, combine the softened butter, honey, rosemary, and lavender in a small bowl. Mix until well combined. Set aside.

Once cooked, remove the steak from the oven and let it rest for 5-10 minutes before slicing.

Slice the steak against the grain and serve with a dollop of rosemary-lavender honey butter on top. Garnish with any remaining grapefruit segments and caramelized onions.