Place the steak in a large plastic storage bag. Whisk together the soy sauce, brown sugar, steak seasoning, minced onion and garlic. Shake to coat. Marinate overnight in the refrigerator.
To prepare, heat a few drizzles of oil in a large skillet. Remove the steak from the marinade and add to the pan. Discard the marinade.
Saute the steak for around 5 minutes or until your desired doneness and browned. Remove from the pan.
Add additional olive oil to the pan. Saute the mushrooms and onions for 5-7 minutes scraping any brown bits from the bottom of the pan. Season with an additional ½-1 tsp of steak seasoning salt and fresh thyme leaves. Adjust seasonings to your taste.
After the mushrooms and onions have browned and the juices released have evaporated add the cream.
Mix thoroughly and add the steak back to the pan cooking just until heated through.
Serve immediately over mashed potatoes or buttered rice.