Put pan on high heat until surface burns. Add olive oil and butter. Chop onions vertically in half and then chop again thin slices. Add onions to the pan and cook until soft and golden brown, for 4-5 minutes. Add sugar, curry and stir. Cut mushrooms in thin slices and add in the pan.
Add salt and pepper, fresh thyme and stir well for 6-7 minutes until all vegetables soften, liquids evaporate, and volume of vegetables is reduced in half.
Take vegetables off the heat and leave aside.

For the steak

Clean the pan with some paper towel and prepare the steak for cooking. Take steak, pour over 2 tbsps olive oil, salt pepper and French mustard and rub with your hands until cover the surface on both sides. Put pan on high heat until surface burns. 
Add steak in the pan and cook each side for 1 minute. Add 1 tbsps olive oil, butter, rosemary sprigs, fresh thyme and garlic.
Using a spoon, pour the liquids from the olive and butter, on both sides of the steak. Take steak out of the pan and put it on a baking dish. Sprinkle grated cheddar cheese on top and bake in the oven at 180C for 2-3 until cheese melts.
Take out of the oven, place on a slice of toasted bread, add some tabasco, cover with another slice of toasted bread and serve!