Steaks -- I like to saute mine in a cast iron pan, but any large heavy saute pan will work for this. If you decide to go with a skirt steak they will not have to be finished in the oven, but if you use a thicker cut of meat, you will need a oven proof pan to finish the cooking in the oven.
Heat the pan to medium high heat and add the olive oil. Season the steak well on both sides with salt and pepper and bring to room temperature. Sear the steak on one side for 4-5 minutes until nice and brown (the time will depend of how thick the steak is) and then flip. I like to finish in a 425 degree oven for about 5-8 minutes until it reaches your desired internal temperature. NOTE: These times are based on on 1 1/2" thick strip or fillet. Remove the steak once done and transfer to a plate and cover with foil to let rest.
Onions -- As the steak finishes in the oven, cook the onions in a separate small sauce pan. Heat to medium high and then add the butter and onions. Season with salt and pepper and saute 5-8 minutes until the onions are lightly golden brown and tender. Finish by adding the thyme. Just reduce to low while the steaks finish and you make the sauce.
Sauce -- After the steak is done and resting, return the pan to the stove on medium heat. Add in the garlic and cook just a minute, then add the white wine to deglaze the pan. Scrape up the drippings as well. Cook just a couple of minutes so it reduces slightly. It doesn't take long. Then remove the pan off the heat as you stir in the brandy and then return it. NEVER pour a flammable liquid like brandy directly over the heat. Return to medium heat and bring to a boil. Then remove from the heat and add in the mustard, butter and blackberry jam. Just whisk everything together. The heat of the pan and wine will warm up everything perfectly.
Serve -- Plate the steak and top with a few of the onions. Then drizzle a little of the sauce on the top. If you go with a skirt steak or something similar you can slice up the steak on a batter and serve it the same way with the onions and sauce.
As I said I love a green salad with blackberries, blue cheese and candied walnuts to bring all the flavors together and my parsnips and cauliflower puree for a nice twist to mashed potatoes. Make the potatoes (aka cauliflower) and the walnuts all ahead, so it is just matter of a quick reheat and the steak. A gorgeous dinner for friends one night.