Heat 1 tablespoon of olive oil in a large saute pan.

Add the sliced onion and ? teaspoon of sea salt. Cook for approx 15 minutes over medium heat until the onion is soft, stirring often to avoid burning.

Meanwhile, cook the pasta according to the packet instructions.

Add 1 tablespoon of butter to the pan, once melted add half the mushrooms and cook until tender. Push the cooked mushrooms to one side of the pan.

Add another ? tablespoon of butter to the pan, once melted add the remaining mushrooms. Cook until they release their juices.

Add garlic to the pan and cook for a minute.

Stir in 1 tablespoon of mustard, ensuring all mushrooms are covered.

Add 2 tablespoons of plain flour to the pan, stir well, and cook for a minute or two.

Slowly add the stock to the pan, stirring continuously. Add 1 tablespoon of Henderson's relish and simmer for a few minutes until the sauce thickens.

Stir in ? teaspoon of freshly ground black pepper.

Turn down the heat to low and stir in the sour cream one tablespoon at a time to make a creamy sauce.

Stir in the chopped parsley, keeping some back for garnish if desired.

Drain the pasta and add it to the mushrooms. Toss gently and serve immediately.