Tip the mushrooms into a pan with the oil and cook for 5 mins until browned. Mix in the flour, then add the Multi mince. Cook until heated through and the sauce is slightly thickened, then tip the mixture into a medium-sized ovenproof dish. Heat oven to 220C/fan 200C/gas 7.
Meanwhile, cook the potatoes in a large pan of boiling salted water for 15 mins until soft. Drain and mash with the buttermilk, then stir in half the spring onions. Spoon over the mince and fluff up. Cook in the oven for 15 mins until the top is lightly browned. Serve sprinkled with spring onions.