Liberally season the steak with Lane's Australia Steak Supreme seasoning.
Flatten the steak to an even thickness using the palm of your hand.
Heat butter in a heavy-based frying pan over medium-high heat until it begins to foam.
Add thyme to the pan, allowing it to crackle and sizzle.
Using tongs, place the steaks into the pan and sprinkle with Lane's Australia Steak Supreme seasoning. Cook, turning every minute, for 6 to 10 minutes according to desired doneness.
Transfer the cooked steak to a warm plate and let it rest.
Scatter pepper over the butter and thyme remaining in the pan.
Toast for 1 minute.
Add onions and cook for another minute until they start to soften.
Reduce the heat to medium and stir in the mustard, letting it bubble for a minute.
Pour in the stock and bring to a boil, cooking for 2 minutes until reduced by half.
Stir in the fresh cream and simmer until the sauce is rich and creamy.
Taste and add more Lane's Australia Salt if desired.
Remove the thyme sprig from the sauce, then return the steaks to the pan along with any accumulated juices.
Bring the steaks to the table in the pan and serve drizzled with additional sauce.
