Mashed Potato Casserole

Bahan-bahan (10 porsi)

• 1.75 kg / 3.5 lb potatoes ((Sebago (Aus), Russet (US), Maris Piper / King Edwards (UK)(Note 1))
• 1 tbsp cooking / kosher salt (, for cooking potatoes )
• 2/3 cup milk ((preferably full fat/whole milk but lite ok))
• 2/3 cup EXTRA milk (, for make-ahead option only (Note 2))
• 75 g / 5 tbsp unsalted butter (, cut into 1cm / 1/2" cubes)
• 1/2 cup sour cream ((or yogurt), full fat best)
• 1 tsp cooking/kosher salt (, or to taste)
• 1 1/2 cups (tightly packed) mozzarella (, freshly shredded (Note 3))
• 1 cup (tightly packed) Red Leicester, cheddar (or other flavoured cheese(Note 3))
• 200 g / 6 oz bacon ((streaky), chopped (Note 4))
• 1/4 cup green onion (, finely sliced)