Pat your steak dry and season generously with sea salt and pepper. Keep the steak out of the fridge so it can come to room temperature. Bring a large pot of salted water to a boil over a high flame. Add the celeriac and boil until completely tender, about 15-20 minutes. Strain and drizzle with 1 tablespoon olive oil along with a pinch of sea salt. Cover and set aside. Meanwhile, heat 1 tablespoon oil in a large sauté pan over a medium flame until hot. Add steak to the hot pan. Cook for 3 minutes without moving. Flip and sear on the other side: 2 minutes for medium-rare; or 4 minutes for medium. When cooked to your desired taste, remove to a plate and let rest for 5 minutes.
Add the broth to the celeriac and purée in a blender or with a hand masher. In a large bowl, toss the baby chard and watermelon radish with the vinegar, remaining oil and a pinch of salt and pepper. Divide the celeriac purée evenly between 2 plates and top with the steaks. Serve with spinach radish salad.