Bring the steak out of the fridge 20 minutes before you cook it, out of the packet covered with paper towels.
Heat the oil in a pan over a medium heat. Fry the bacon pieces until golden and crispy, then set aside on paper towels. Drain the excess fat from the pan, reduce the heat to medium-low and add the shallots. Sweat for 5 minutes, then add the garlic and cook for another couple of minutes.
Add the mushrooms and thyme, and cook for about 7-10 minutes until soft and fragrant. Add the wine to the pan, turn up the heat and let it bubble for 30 seconds to evaporate the alcohol. Add the chicken stock and porcini powder (if using), let everything simmer away to reduce by two thirds.
Add the cream and mustard. Simmer for another 5 minutes or until the sauce is a nice consistency for pouring on to a steak and coats the back of a spoon. Add the chives. Taste and season with sea salt and pepper.
Brush the steaks all over with oil and season generously with salt and pepper. Heat a splash of oil in a large heavy-based frying pan over a high heat and when the pan is hot, cook the steaks until medium-rare or until your liking. Transfer to a clean board, cover loosely with foil and rest for 5-10 minutes before serving. Tip any resting juices into the sauce at the end, mix to combine, and reheat the sauce briefly just before serving. If need be, add a little more cream/stock if the sauce is too thick.
Serve the steaks with the sauce spooned on top and parsley sprinkled over. Serve with a green salad for a lighter meal, or mash and green beans.