Layer seasoned ground beef and mushrooms under a cheesy mashed-potato topping, then bake until golden.

Steps

1. Peel and halve potatoes. Cook in salted water until tender, or use leftover potatoes.
2. In a skillet, heat 2 tbsp of the oil; cook onions and garlic for 5 minutes or until softened.
3. Add mushrooms and cook for 2 or 3 minutes or until golden. Transfer to a shallow baking dish and set aside.
4. In the same skillet, cook beef about 5 minutes or until broken up and evenly browned. Drain off fat and add meat to mushrooms.
5. In the same skillet, bring stock and tomato paste to the boil, stirring constantly. Remove from heat and stir in mustard, thyme, 1/2 tsp salt and a pinch of pepper. Add to meat mixture and stir well.
6. In a clean skillet, heat remaining oil and peppers for 2-3 minutes; sprinkle over meat mixture.
7. Mash potatoes; blend in hot milk, egg yolks, cheese and salt. Beat egg whites until stiff and fold into potato mixture. Spread over meat mixture.
8. Bake at 425°F for 5 minutes. Reduce heat to 375°F and bake for 20 minutes or until golden brown.