Photograph: Laura Edwards
Preheat the oven to 200°C, fan 180°C, gas 6. Bake the potatoes until tender, about 1 hour 15 minutes. Soak the mushrooms in a jug of boiling water. Heat the oil in a large pan and cook the onion, carrot, celery and thyme until softened, then transfer to a plate. Brown the mince in the pan, breaking it up with a spoon.
Return the vegetables to the pan; stir in the tomatoes, peas and consommé. Drain the mushrooms (reserve the liquid), chop and add to the pan with 100ml of the liquid. Season and simmer for 30 minutes, stirring occasionally.
Warm the milk with the spring onions. Halve the baked potatoes and scoop out the centres into a large bowl. Add the butter and mash until smooth. Stir in the milk, spring onions and half the cheese. Season to taste.
Check the seasoning of the mince, discard the thyme sprigs, then pour into a deep 20cm-square ovenproof dish. Top with the mash and smooth over. Toss the thyme leaves and breadcrumbs with the remaining cheese and scatter over. Bake for 25-30 minutes in the preheated oven until golden and bubbling.
Get ahead: cook and cool the mince and the potato, then assemble the pie the day before; chill. Bring back to room temperature before baking. Can be frozen.