Put a nice dollop of olive oil in a pan and fry leek and mushrooms until the leek is caramelised.
Add stock, Guinness, tomato paste, tarragon and corn flour to the pan and stir to combine all of the ingredients. Ensure that the corn flour is added while the sauce is still relatively cool. Otherwise you'll get a lumpy sauce. Bring to a heat where the sauce is simmering. Reduce sauce until you have the consistency of a gravy. Stir regularly. Takes around 30 minutes.
Around 15 minutes into reducing your sauce, depending on how you like you steaks, put the steaks into a pan and cook to your liking. I usually put olive oil in a pan and bring to a high heat, lightly salt the steak and then add to the pan to get a nice sear on it then immediately reduce the heat. Then Cook on the one side without turning until juices rise on top then turn up the heat and flip the steak to give a nice sear on the other side for around 20 - 30 seconds. I then put the steaks aside to rest.
once sauce has a nice consistency of gravy I put the steak on the plate and put the mushroom sauce over the top. Enjoy.