Melt the butter with the olive oil in a big, wide saucepan, add the onion, garlic and mushrooms and sauté gently for 10 minutes.
Sprinkle in the flour and mix well until it has absorbed the oil and butter, then gradually add the red wine and mix until smooth. Season. Bring to the boil. Add the salmon fillets to the pan and spoon the red wine over them to stain them evenly. Cover, then simmer gently for 8 minutes.
Lift out the salmon and keep it warm. Bring the wine back to the boil, mix well, then with a slotted spoon lift out all the mushrooms and serve them with the salmon.
Don’t discard the wine – it can be stored in the freezer and re-used.