Sprinkle the steak with the Cajun spice mix and seal in 2 tablespoons of olive oil in a hot frying pan for 5-10 seconds on each side. Lay steak to one side.
Prepare the onion crust by cooking the onions in the pan containing the residue of the steak. Add a knob of butter and cook the onions until caramelised.
In a bowl combine the onions, mustard, egg yolks, tarragon and enough of the flour to turn the mixture to a paste-like consistency.
Lay spoonfuls of the onion mixture on the top of each steak and spread to cover the meat.
Put 2 tablespoons of oil in a pan and heat.
Place the steaks, onion-side down into the pan for a few seconds to seal the onion crust.
Cook the steaks in an oven, preheated to 200 C for about 6-7 minutes for a medium-rare steak: longer for a more well done steak.
Alternatively the steaks can be cooked in a frying pan.
Serve with saut, ed mushrooms.