Heat the oil and first measure of butter in a large frying pan over medium heat. Add the onion and mushrooms and cook for 8-10 minutes until softened and lightly caramelised. Add the garlic and cook for a further 30 seconds until fragrant. Transfer to a plate.
Increase the heat to high. Season the sliced beef lightly with salt and pepper, then add it to the pan in a single layer. Cook for 2-3 minutes until browned but still slightly pink inside.
Pour in the brandy, if using, and cook until almost evaporated. Add the beef stock, Worcestershire sauce and wholegrain mustard, scraping up any browned bits from the base of the pan. Return the mushrooms and onion to the pan and simmer for 3-4 minutes.
Remove the pan from the heat. Stir a few spoonfuls of the hot sauce into the Greek yoghurt, then gently stir the yoghurt mixture through the stroganoff until combined. Return to very low heat for 1-2 minutes until warmed through, but do not allow it to boil. Finish with the remaining butter, stirring through until melted and incorporated.
Season to taste with salt and freshly ground black pepper. Scatter with chopped parsley and serve immediately with steamed rice, pasta or mashed potatoes.
