1. Place potatoes in large saucepan and add enough water to cover. Bring to a boil. Reduce the heat to medium-low, cover and simmer for 15 to 20 minutes or until easily pierced with a fork. Drain well.
2. Transfer potatoes in a bowl and add the ingredient A (butter, sugar and salt), then mash the potatoes until creamy with a potato masher or fork. Place the mashed potatoes into your ovenware. Set aside.
3. Make the beef topping : In a skillet, heat 1 tbsp olive oil and fry the garlic until fragrant. Add onion, beef and mushrooms, continue stir frying until it becomes wilted. Add the ingredient B to season and simmer for a few minutes.
4. Add sour cream and mix, stir for a few more minutes.
5. Preheat the oven to 200C° degree.
6. Sprinkle cheese over the mashed potatoes and pour the beef mixture over the cheese.
7. Bake in the preheated oven for 30 to 35 minutes.