Sprinkle the steak with 1 tablespoon of the berbere spice and let stand at room temperature for 1 hour.
Meanwhile, preheat the oven to 350°. On a large rimmed baking sheet, toss the tomatoes with the olive oil. Season with salt and pepper and bake for 45 minutes, until the tomatoes are sizzling and just starting to brown.
In a medium bowl, combine the lemon juice with the remaining 1/2 tablespoon of berbere spice. Add the celery and leaves, the parsley, capers, vinegar and onion and toss well. Season with salt and pepper.
In a large skillet, melt the butter. Season the steak with salt and pepper and cook over moderately high heat until richly browned, 5 minutes. Turn the steak and cook over moderate heat until medium-rare, 6 minutes longer. Transfer the steak to a carving board to rest for 10 minutes.