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Heat 1 tbsp butter, the rosemary and garlic in a frying pan over a medium heat. Once bubbling, toss in the mushrooms and cook until golden and tender. Add the remaining 1 tbsp butter to the pan with the yeast extract and swirl it around to coat the mushrooms. Remove from the heat and scatter over the parsley with a little seasoning.
Divide the mushrooms between the 4 slices of toast, and drizzle over any butter left in the pan. Serve immediately.