For the salsa verde:
Finely chop the garlic, capers, gherkins, anchovies and herbs and put them into a bowl. Add the mustard and vinegar, then slowly stir in the olive oil until you achieve the right consistency. Balance the flavours with freshly ground black pepper, a bit of salt and maybe a little more vinegar.
Tie up the stalk ends of your bunch of thyme, place the leafy ends in a pestle and mortar and give it a good bash. This will remove the tasty leaves and leave you with what looks like a miniature broom. Put this to one side. Add your garlic, anchovy fillet and lemon zest to the mortar. Bash to a paste and stir in your olive oil.
Your fillet steaks should be about 2.5cm thick. Wrap a piece of bacon around each steak (this gives them a really good flavour) and secure loosely with a piece of string. Peel the mushroom skins off, which only takes a second and helps them absorb the marinade. Brush the steaks and mushrooms with some of your flavoured thyme oil, keeping the rest for the cooking.
Try to become instinctive about cooking basic things like steaks and to understand heat, sizes and cuts of meat. Chefs test their meat by the way it looks and the resistance it gives when squeezed. Preheat your griddle pan or barbecue until really hot. I don't want to give you a specific time to cook them, as your steaks may be thicker or thinner than the ones I'm using. Whether using fillets or sirloins, I cook mine for roughly 3 or 4 minutes each side and let them rest for a couple of minutes to give me a medium-cooked steak – you can do them for a little more or less time to your preference.
Season the meat on both sides and place on the griddle pan with your mushrooms. Turn every minute, and brush each time with your thyme oil brush. The mushrooms will be cooked after about 6 minutes and should be soft to the touch – cooking them this way means they don't go all soggy but they do have an intense meaty flavour. Once the beef is cooked to your liking, remove the string, divide the steaks between 4 warmed plates with your mushrooms, allow to rest for 2 minutes and put a big dollop of salsa verde over the top.