Heat the oven to 425F with the rack in the lowest position. On a large rimmed baking sheet, toss the potatoes with 2 tbsp olive oil, ¼ tsp salt, and black pepper to taste and spread in a single, even layer. Roast 25-30 minutes, stirring once halfway through.
Meanwhile, prepare the dressing: Whisk together the olive oil, balsamic vinegar, and dijon mustard until emulsified. Season with salt and pepper.
Then, prepare the Steak: Whisk together the spices in a small bowl. Pat the steak dry all over, and rub the spice mixture into the meat so that the entire surface (both sides!) is covered. Heat a large cast iron or steel pan over high heat. When hot, add the oil, followed by the steak. Sear 4 minutes per side for medium-rare (an internal temperature of 130-135F). Transfer to a cutting board and allow to rest 10 minutes. Slice the steak into thin strips across the grain.
Gently toss the baby lettuces with the dressing and plate topped with the steak, green bell pepper, red onion, blue cheese, and tomatoes.