Heat oil in a pan, add steaks and sear 2 minutes each side, cook for a further 4-5 Min's a side.
Remove to an oven proof dish and cook for a further 15-20 Min's for Medium rare-Medium. Please adjust cooking times for thinner steaks as I said above mine were about 300g a piece and this cooking time gave us a tender steaks, nice and pink in the middle.
Melt butter in a pan, add mushrooms, cook, until almost tender, add garlic and cook for a further 2 Min's, stirring, so garlic does not burn.
Add creme fraiche, cream and stock, bring to the boil. Reduce heat to a simmer, mix a little water with cornflour, add to sauce and stir until mixture thickens.
Stir in onions and parsley, serve sauce over steak.