Score both sides of each steak by making diagonal slices. Season with barbecue seasoning. Brush each side of the steak with the mustard, then press 1/2 of the red onions into the mustard on one side of the steak.
Melt the butter in a large frying pan over high heat. Place the steaks onion side down in the frying pan. Press the remaining onions evenly into the mustard on the top side of each steak.
Cook steaks until they start to firm and are browned on the outside and red in the centre, 4 to 5 minutes per side. An instant-read thermometer inserted into the centre should read 52 degrees C. Remove steaks from the frying pan and allow them to rest for 5 minutes before serving.