Heat the oil and add the onion and thyme.
Sauté until translucent, reduce heat a little and allow the onion to caramelise gently for about 10 minutes, stirring occasionally.
Increase heat and add the stock and wine.
Bring to a boil, reduce heat, cover and simmer the sauce for 10 minutes.
Now increase heat and cook rapidly without the lid until the sauce has been reduced by half.
Stick your stick blender into the sauce and whizz until smooth.
Whisk in the mustard and add the peppercorns, keeping them whole.
If the sauce is not thick enough to your liking, mix the optional flour with the melted butter and whisk into the sauce.
If the sauce is thick enough for you, then whisk in just the butter.
Remove from the heat and set aside, covered.
Tie each steak with string, if you like, to retain its shape otherwise just pat them down a bit.
Drizzle generously with olive oil and rub all over.
Now, sprinkle each steak with the mustard powder: not to worry, the mustard loses its sting in the grilling process but adds seasoning and flavour to your steak.
Now grill the steaks to your liking – over the coals or in your griddle pan – and when done, transfer to a board, resting them for at least 5 minutes.
Serve with garlic and parmesan mashed potatoes and a green vegetable salad.
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