Add the salt, pepper, onions, garlic and rosemary.
Stir it up and let it cook until the onions just start to brown a little on their edges and add the butter and the red wine. Stir with a wooden spoon scraping the bits of meat off the bottom.
Put the steaks and the drippings back in and turn them over a few times as the butter melts and the wine reduces. When the filet mignon is cooked to the degree of doneness that YOU want – rare is soft to a poke or 140-145° on a meat thermometer – starting to get firm or 155-160° is medium – firm or 165-170° is well done.