Place peeled and cubed Yukon Gold potatoes into a large 6-quart stockpot. Cover with cold water by at least 1 inch and stir in 1 generous tablespoon of kosher salt. Bring to a rolling boil over high heat, then reduce to medium-high and simmer for 15 to 18 minutes until potatoes are fork-tender and easily pierced with a knife.

Drain potatoes thoroughly in a colander. Return the hot drained potatoes back into the warm dry pot set over very low heat for 1 to 2 minutes, shaking gently to steam off excess surface moisture (this is the chef secret for maximum fluffy creaminess).

Remove pot from heat. Add the mashed roasted garlic paste, softened cream cheese, and cubed butter. Using a potato masher or potato ricer, mash until creamy and well incorporated.

Gradually pour in the warm heavy cream and whole milk while stirring with a sturdy wooden spoon. Fold in 1/2 cup grated Parmesan cheese, minced chives, chopped thyme, rosemary, 1 1/2 teaspoons kosher salt, and cracked black pepper. Whip until velvety, smooth, and cloud-like (taste and add a pinch more salt if needed).

Preheat oven to 375°F (190°C). Butter a 9x13-inch (or 3-quart) ceramic baking dish. Spoon the whipped mashed potatoes into the dish, creating rustic swoops and swirls with the back of a large spoon.

In a small bowl, toss together the panko breadcrumbs, shredded Gruyere, 1/4 cup grated Parmesan, and 2 tablespoons melted butter until evenly moistened. Sprinkle the herb crumb mixture evenly over the potatoes.

Bake uncovered for 25 to 30 minutes until the casserole is heated through and the edges are bubbly. Switch to BROIL on HIGH for 2 to 3 minutes until the breadcrumb crust turns deeply golden brown and crisp. Garnish with remaining chopped fresh chives and a drizzle of melted butter. Serve warm!