Peel potatoes and rinse in cold water (cut potatoes in quaters if very large)
Preheat Oven 400 F / 200 C.
Cook Potatoes - Transfer them to a large stockpot filled with cold water. THe water needs to be 1 inch above the potatoes. Then turn the heat to high and cook until the water comes to a boil. Reduce heat to medium and continue cooking for 12-15 minutes until the potatoes are fork tender. Drain the potatoes immediately.
Mash Potatoes- While still hot, run potatoes through a potato ricer or transfer to the bowl of your stand mixer. Using the whisk attachment on low speed mash the potatoes. You can also use a potato masher instead.
To the mashed potatoes add softened butter, milk, garlic powder, half of the Gruyere cheese, salt, and pepper. Stir everything together using a spatula or wooden spoon until combined. The potatoes should be creamy and fluffy. Don't overmix or else the potatoes will turn gummy.
Spread half of the potatoes in the baking dish. Sprinkle remaining cheese. Layer with rest of the mashed potatoes. Smooth the surface with back of a spatula.
Bake for 20 minutes until the top is light golden brown in colour and lightly crispy. The cheese will start bubbling around the edges.
While potatoes are in the oven, melt butter in a pan. Cook on low heat until the butter is turning light golden yellow in colour and smells like caramel. Once it is just light golden yellow in colour, add sage leaves. Take it off the heat (butter and sage will continue cooking further in the remaining heat, so don't get tempted to cook longer).
Take potatoes out of the oven, spoon brown butter along with crispy sage leaves on top. Serve.