Ground beef and mashed potatoes gratin

Ingredients

For mashed potatoes

• 1kg potatoes - peeled, cut into quarters
• - Ingredient A -
• 40g butter
• 1 tsp sugar
• 1/2 tsp fine salt

For the topping

• 1 tbsp olive oil
• 350g ground beef
• 2 garlic cloves - finely chopped
• 1/2 onion - finely chopped
• 160g brown mushroom - thinly sliced
• - Ingredient B -
• 2 tsp soy sauce
• 1 tsp kosher salt
• 1 tsp sesame oil
• 1 tbsp mayonnaise
• 1 tbsp tsuyu
• 1/2 tsp sugar
• 320g sour cream
• 50g or more (as your choice) grated cheese