Heat 1 tablespoon of oil and the butter in a non-stick frying pan over low heat. Add the onion and thyme and cook for 10 minutes or until onion is softened. Add the sugar and balsamic vinegar and cook for a further 8-10 minutes or until caramelised.
Heat the remaining oil in a non-stick frying pan and cook the steaks for 1-2 minutes each side or until cooked to your liking. Spread ½ a tablespoon or horseradish cream over each baguette half, top with the steak and cheese.
Place under a preheated hot grill and cook for 2-3 minutes or until cheese is melted and golden. Top with caramelised onion and rocket to serve.