Preheat oven to 400 degrees and remove the steak from the refrigerator and allow to to come to room temperature. Strip the thyme leaves off the sprig. Quarter the tomato and toss on a baking sheet with 1/2 tablespoon olive oil, the thyme leaves, and a pinch of salt and pepper. Place in the oven to roast for 20-25 minutes, until softened and slightly caramelized.

Meanwhile, halve, peel, and thinly slice the red onion. In a small bowl, combine the sour cream with 1 teaspoon mustard (or to taste), horseradish powder, and a pinch of salt and pepper. Set aside.

Heat 1/2 tablespoon olive oil in a medium pan over medium heat. Season the steak with salt and pepper on both sides. Sear the steak for 6-7 minutes per side, until browned and cooked to desired doneness. Remove from the pan and set aside for 5 minutes to rest.

While the steak breasts, add the red onion to the same pan you cooked the teak in. Cook, tossing, over the medium heat for about 5 minutes, until softened. Stir half the watercress and half the balsamic vinegar into the onions and cook for about 1 minute, until watercress is slightly wilted. Season with salt and pepper. Meanwhile, halve each ciabatta and place them on the baking sheet in the oven to toast for about 5 minutes.

Remove the baking sheet from the oven. Thinly slice the steak against the grain. In the medium bowl, toss the remaining watercress with 1/2 tablespoon olive oil, remaining balsamic, roasted tomatoes, salt, and pepper.

Spread the horseradish cream on the toasted ciabatta. Top with the sliced steak and wilted watercress mixture. Serve alongside the watercress-tomato salad. Enjoy!