Preheat an outdoor grill for high heat and lightly oil the grate.
Mix 1 teaspoon salt, cumin, coriander, and garlic powder together in a small bowl. Sprinkle seasoning over steak. Add 1/4 teaspoon pepper.
Pour 1 tablespoon oil into a large bowl; grate garlic into the bowl. Add potatoes, 1 teaspoon salt, and 1/2 teaspoon pepper. Toss well to combine.
Place broccolini on a platter. Drizzle 1 tablespoon oil on top. Season with salt and pepper.
Lay steak on the hottest surface of the grill. Spread potatoes into 1 flat layer next to the steak, leaving some room for the broccolini. Close grill and cook for 5 minutes.
Whisk sour cream, blue cheese, and Worcestershire sauce together in a bowl. Season sauce with salt and pepper.
Flip steak and potatoes over with tongs. Spread broccolini and lemon onto the grill; cook until broccolini is lightly browned, about 2 minutes. Flip. Close grill and cook until steak begins to firm and is hot and slightly pink in the center, about 3 minutes more. An instant-read thermometer inserted into the center should read 140 degrees F (60 degrees C).
Remove steak and let rest, covered, on a flat work surface, about 5 minutes. Transfer broccolini and potatoes to a separate platter. Slice steak. Divide slices between 4 plates; arrange broccolini, potatoes, and blue cheese sauce on the side. Squeeze grilled lemon halves on top.