Season steaks with salt and pepper. Heat butter in a heavy skillet over medium-high heat. Sear steaks until browned -- about 3 minutes. Flip steak and cook opposite side just until browned. Don’t overcook or you run the risk of toughening the meat. Remove steak from pan and set aside.
Place radicchio wedges and lemon slices in the same pan and sear until lemonds are browned and and radicchio has begun to wilt. Drizzle with balsamic vinegar. Slice steak into this strips. Toss arugula with lemon juice, 1 tablespoon olive oil and flaky salt. Plate arugula and top with charred radicchio, steak and tangelo segments. Squeeze charred lemon over salad and serve.