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.