I know it sounds like a really odd combination, but it turns out to be delicious. The blackberries give the soup a fantastic kick. I've also heard of blueberries being used instead of the blackberries, but I haven't tried it yet.
Heat the oil in a large saucepan. Add the steak and cook over medium-high heat, browning it on both sides, then remove it and set it aside.
Turn the heat down to low and add the butter and onions to the pan. Stir frequently, scraping up the meat juices to mix with the onions. Cook for about 10 minutes, until the onions are soft.
Add the broth and bring to a boil, still stirring frequently. Add the mashed berries and honey and simmer for about 20 minutes. Meanwhile, cut the steak into bite-sized strips. Add the steak to the soup near the very end of cooking and allow to simmer for about 30 seconds or so, stirring constantly, then serve hot.