Preheat a grill or a grill pan to medium-high. Drizzle the steaks with EVOO and sprinkle with rosemary. Let them hang out for 15 minutes; season both sides with salt and pepper. Grill the steaks for 3 minutes on each side for medium-rare, then remove from the heat and let rest for about 5 minutes.
While the meat mellows, start in on the salad: Thinly slice the celery at an angle and pile on a platter. Halve and thinly slice the portobellos, then toss them with the celery, using your fingers. Scatter the parsley and cheese evenly on top of the salad, then dress the salad liberally with the lemon juice, EVOO, and salt and pepper.
Thinly slice the rested steaks, then arrange the slices across the top of the salad.