In a heavy skillet over medium-high heat, add 1 tablespoon olive oil and heat until very hot. Season the steak with salt and pepper. Turn on the exhaust fan and put the steak in the pan; stand back to prevent being splattered. Cook until the steak is brown on the first side, about 2 minutes; brown on the other side. Remove the steak to a rack or baking pan until you are ready to finish cooking it. When ready, put the steak into the oven and cook until done to your liking, about 10 minutes for rare. Remove the steak to a carving board and let it rest for 5 minutes.
Peel and cut the onions into thirds or quarters, depending on their size. Leave as much root on as possible to keep the wedges from falling apart. Heat 1/4 cup olive oil in the same skillet used to cook the meat over medium-high heat until hot. Add the onions, reduce the heat to medium, and cook until brown on all sides, about 5 minutes. Season them with salt and pepper. Place the pan of onions in the oven and roast until tender and very browned (the onions tend to char around the edges), 15 to 20 minutes.
Remove the onions from the oven, add the garlic, and cook briefly until light brown. Add the thyme and stir. Add the vinegar (stand back so as to not get splattered) and toss well with the onions. Stir and scrape up all the browned bits clinging to the bottom of the pan.
To serve, cut the meat off the bone, then carve it into thin slices. Put 1 slice of meat on each crostini and top with the roasted onions. Pour any juices from the carving board over the crostini. Serve immediately.
Chef's Note: In order to cook the meat in the shortest amount of time and thus extract the most flavor, the meat must be at room temperature. Afterward, the meat must rest before carving. If this procedure is followed, the carving knife glides right through the steak.
A 1 1/2 pound porterhouse should give you about 14 ounces of meat off the bone, enough for 3 or even 4 people. However, if you are steak lovers, you might want to buy 2 steaks of that size for 4 people.
1 loaf crusty bread, sliced into 1/2-inch thick slices
Extra-virgin olive oil
Salt and freshly ground black pepper
Preheat the oven to 400 degrees F.
Place the bread in a single layer on a cookie sheet. Drizzle with olive oil and sprinkle with salt and pepper. Bake until the bread is golden brown and crisp, about 8 to 9 minutes.