Halve any large onions, leave the rest small and put in a small saucepan with the garlic and stock. Simmer for 10 minutes, then drain, reserving the liquid.
Heat the butter with 1 tablespoon of the olive oil in a frying pan and fry the mushrooms until they are golden brown and no liquid remains; this will take a good 10 minutes. Remove from the pan with a slotted spoon and set aside.
Add the onions and garlic to the pan. Stir over the heat for 2-3 minutes until golden, then sprinkle in the flour. Cook, stirring, for 1 minute before adding the port, conserve and 100ml of the reserved cooking liquid. Bring to the boil and bubble for 2-3 minutes until reduced and sticky.
Return all the mushrooms to the pan, add a little lemon juice to taste with plenty of seasoning and set aside.
Brush the steaks with oil and season well. Heat a large frying pan or griddle pan over a high heat for 2 minutes, then cook the steaks for 3-4 minutes on each side for medium rate (adjust the cooking time for rare or well done). Transfer the steaks to a warm plate and leave to rest for 5 minutes while you reheat the sauce.
Pour any steak juices from the pan into the wild blueberry sauce, reheat gently for 1-2 minutes, then serve with the steaks. Accompany with crispy frites and a leafy watercress salad.
The sauce can be made the day before it's needed, but bubble to reduce for 1-2 minutes only. Keep the fried mushrooms separately in the fridge. When ready to use, bring the sauce to the boil and simmer gently to reheat and reduce further. Stir in the mushrooms and lemon juice as above.
Make sure you get a good golden colour on the mushrooms, onions and garlic for the best flavour and colour in the finished sauce.
