Preheat the oven to 350°F.

Rub the bottom of a 9 x 13-inch baking dish with softened butter.

Combine the half-and-half and the cream in a large measuring cup.

Using a mandolin or a very sharp knife, slice the potatoes very thinly. They'll cook better that way.

Place the potatoes in a large bowl.

Drizzle with half of the half-and-half/cream mixture and set aside.

In a large skillet over medium-low heat, melt the butter.

Add the garlic and onion and cook until translucent, stirring occasionally, about 4 minutes.

Cut the bar of cream cheese in half and add the halves to the skillet, stirring constantly. Continue stirring until the mixture is smooth and combined, about 3 minutes.

Pour in the remaining half-and-half/cream mixture and stir to combine.

Add salt and pepper and stir. Taste and adjust the seasonings as necessary. Do not under salt! Add the rosemary and green onions and stir to combine.

Finally, add ½ cup of the grated parmesan and stir to combine.

Pour the cream-soaked potatoes into the baking dish. Pour the cream cheese mixture over the top and spread it evenly over the potatoes. Scrape out the skillet to get every last drop.

Sprinkle the remaining 1/2 cup grated parmesan generously over the top and bake the potatoes for at least 1 hour, or until golden brown and bubbly.

Remove from the oven and let stand for 10 minutes.

Sprinkle with the chives, then cut into squares to serve.