Halve onion and thinly slice. Drain tomatoes, reserving puree, and finely chop. Wearing protective gloves, seed chile, if desired, for a milder sauce and finely chop. Cut steak crosswise into 4 pieces. Pat steak dry and season with salt.
Heat a dry 12-inch heavy skillet over moderately-high heat until hot and add oil. Saute steak about 2 minutes on each side for medium-rare and transfer to a baking dish. Keep steak warm in oven.
To skillet add the onions with tomatoes, reserved puree, chile, and salt to taste and simmer, stirring occasionally, until sauce is thickened slightly and onion is crisp-tender, about 5 minutes. Stir in any juices that have accumulated on baking dish from steak. Serve steak with sauce.