Preheat oven to 180°C (Gas Mark 4, 350°F). Heat the olive oil in a large frying pan and add the onions and mushrooms. Cook over a high heat until starting to soften and caramelise before adding the garlic. Cook, stirring, for 2 minutes further.
When the mushrooms and onions have softened, add the flour to the pan and cook for another couple of minutes before adding the rest of the filling ingredients and stirring until bubbling and thickened.
Check the seasoning of the gravy, adjust if necessary and divide the filling between the ramekins.
Cut discs from the sheet of puff pastry to fit the ramekins and pop them on top of the filling.
Decorate with any leftover pastry, place the pies in a roasting tin and transfer to the oven to bake for 30 minutes. When cooked, the pastry should be golden and risen and the filling bubbling hot.
Serve immediately, with mashed potatoes and green vegetables.