Heat the butter in a frying pan, add the mushrooms and cook for 2 minutes. Add the garlic and cook for a further 1 minute until the mushrooms are lightly golden. Stir in the sherry and cook until evaporated, then season.
Meanwhile, drizzle the oil over the bread and toast in a preheated griddle pan for 1–2 minutes on each side until golden and crisp.
Spoon the mushrooms over the toast, then scatter with the parsley and Manchego cheese shavings to serve.