Place a rimmed baking sheet in the oven and preheat to 450˚. Cut the cauliflower in half through the core, place cut-side down and slice through the stem into long, 3/4-inch-thick florets. Gently toss the cauliflower and any leaves with 2 tablespoons olive oil in a large bowl; season with salt and pepper. Spread out on the hot baking sheet and roast until browned and tender, about 30 minutes.
Meanwhile, evenly pound the flank steak with the textured side of a meat mallet until 1/2 inch thick. Halve the steak crosswise and lengthwise; rub with salt and pepper, the paprika and granulated garlic. Let sit at room temperature 10 minutes.
Thinly slice the celery and toss with the vinegar in a medium bowl; let sit 5 minutes. Chop the roasted red peppers, olives and parsley and add to the celery along with 2 tablespoons olive oil, 2 pinches of salt and a few grinds of pepper; toss.
Heat the remaining 1 tablespoon olive oil in a large nonstick skillet over medium-high heat. Add the steak and cook until browned, 3 to 4 minutes per side for medium rare. Transfer the steak to a cutting board and let rest 5 minutes, then slice against the grain.
Divide the steak and cauliflower among plates. Spoon the red pepper salsa on top.