Preheat your oven to 375°F. Grease a 9×13-inch casserole dish with butter.
Place the potatoes in a Dutch oven and cover them with water. Bring to a boil over high heat and cook until tender, about 10 to 15 minutes. Drain well and return the potatoes to the hot pot.
Using a hand mixer or potato masher, mash the potatoes with butter, cream cheese, sour cream, onion powder, garlic powder, thyme, smoked paprika, cayenne pepper, Monterey Jack cheese, salt, and pepper. Gradually add the milk and beaten eggs, mixing gently to combine without overworking the potatoes.
Transfer the mashed potatoes into the prepared casserole dish, spreading evenly.
To prepare the topping, melt the butter in a skillet over medium heat until it starts to brown, about 2 to 3 minutes. Remove from heat and stir in the Panko breadcrumbs, crushed corn flakes, garlic powder, and a pinch of salt and pepper. Evenly sprinkle this mixture over the potatoes, then top with grated Gruyère cheese.
Bake in the preheated oven until the topping is golden brown and crispy, 30 to 40 minutes. Garnish with fresh parsley or thyme before serving. Enjoy!