Cut the fat off the steaks and discard. Slice the steaks very very thinly against the grain of the meat. (See notes below and above.)
Heat the 1tsp oil in a large frying pan or saute pan and add the steak strips. Cook for 3-5 minutes until browned all over.
While the steak is cooking, cut the bread in half lengthways and toast lightly on both sides under a hot grill. Set aside.
Remove the steak from the pan and set aside.
Add the remaining oil (no need to clean the pan) and the onions and peppers, cook for 5 minutes over a high heat until they are just starting to blacken at the edges.
While these are frying, add the stock cube to the 100ml/0.5 cup boiling water and stir until melted.
Pour the stock, Worcestershire sauce and some black pepper into the pan and stir everything (it may bubble furiously!) Let it bubble for a minute or too just until the liquid is nearly evaporated (this will depend on how fierce your hob is). Remove from the heat.
With the BOTTOMS of the toasted baguettes on a grill safe baking tray, top with the onion mixture, then the steak and finally with the cheese. (Optional: sprinkle with chilli flakes.)
Put them under the grill for 3-5 minutes or until the cheese is melted. Add the tops to sandwich together and serve.