Preheat oven to 250 degrees F.
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.