Heat the oil in a large pan and cook the onion, carrot and celery over a medium heat for 10 minutes until soft. Add the garlic for a final minute.
Add the mince and break it up, cooking for 8–10 minutes until well browned. Take your time — this is where the flavour comes from.
Stir in the flour and tomato paste and cook for a minute, then add the stock, Worcestershire sauce, Marmite and thyme.
Simmer uncovered for 25 minutes, stirring occasionally, until thick enough that a spoon dragged through the pan leaves a track. Stir in the peas, season well, and take off the heat to cool for 10 minutes.
Meanwhile, boil the potatoes in salted water for 15–18 minutes until tender. Drain and leave in the colander for 2 minutes to steam dry.
Mash with the butter and just enough warm milk to make a stiff, smooth mash. Season well. It should hold its shape on a spoon.
Heat the oven to 200°C (180°C fan). Spoon the filling into a 2.5 litre baking dish.
Spoon the mash on in blobs around the outside edge first, then fill in the middle and join it all up with the back of the spoon. Rake the surface with a fork and scatter over the cheese if using.
Bake for 30 minutes until the top is golden and crisp at the peaks and the filling is bubbling at the edges.