Peel the potatoes and cut them into large chunks similar in size so they cook evenly. Place the chopped potatoes in a large pot over high heat and cover with water about 2 inches above them. Add salt, fresh thyme, and garlic and stir briefly to combine.
1.2 lb potatoes, 3 garlic cloves, 3 springs fresh thyme, Salt and black pepper to taste
Preheat the oven to 400F /200C.
Reduce the heat to medium-high and boil the potatoes until fork tender, about 12-15 minutes. Drain the potatoes completely (remove the thyme stem) and return them to the pot. Let them cool while you heat the milk and butter.
In the meantime, Heat the milk and butter in a saucepan until the butter melts. Remove the pan from the heat and set aside.
6 Tablespoons unsalted butter, 1½ cups Milk
Using a potato ricer, handheld masher or electric potato masher, mash the potatoes until smooth and creamy.
Slowly pour in the milk and butter mixture, stirring continuously, until well combined. Next, add chopped parsley and season with salt and pepper to taste.
Add in the shredded mozzarella, gruyere, and cheddar cheese (reserving some to top the casserole later) and gently fold it into the mashed potatoes.
1 cup sharp cheddar cheese, ½ cup mozzarella cheese, ½ cup gruyere cheese
Transfer the mashed potato mixture to a casserole dish and spread the top evenly. Lastly, top with the more shredded cheese.
Bake uncovered in the oven until the cheese is melted, bubbly and golden brown, about 20-25 minutes. For a more golden brown crust on top, switch to broil for the last few minutes of baking.
Remove the baking dish from the oven and leave to cool slightly.
Serve hot as a side dish with your favorite protein, vegetables and gravy for a complete meal.