Prepare the mushrooms. Clean them gently with a damp cloth (avoid soaking them in water). Slice them into even pieces, about 5mm thick. This ensures they cook evenly and release their flavour properly.
Sauté the onions and garlic. Heat the olive oil and butter in a large pan over medium heat. Add the chopped onion and cook until softened and translucent, about 5-7 minutes. Then, add the minced garlic and cook for another minute until fragrant. Be careful not to burn the garlic, as it can become bitter.
Cook the mushrooms. Add the sliced mushrooms to the pan and cook, stirring occasionally, until they are softened and browned. This usually takes around 8-10 minutes. The mushrooms will release their moisture, so let it evaporate while they cook. This step is key for developing that rich mushroom flavour.
Make a roux. Sprinkle the flour over the cooked mushrooms and stir well to coat everything evenly. Cook for 1-2 minutes, stirring constantly. This creates a roux, which will help thicken the sauce. It’s important to cook out the flour to avoid a floury taste.
Add the stock. Gradually pour in the beef stock, stirring constantly to avoid any lumps forming. Bring the mixture to a simmer and cook for 5-7 minutes, or until the sauce has thickened slightly.
Finish the sauce. Stir in the double cream, Worcestershire sauce, and thyme. Season with salt and freshly ground black pepper to taste. Simmer gently for another 2-3 minutes, allowing the flavours to meld together. Be careful not to boil the sauce after adding the cream, as it can split.