Preheat the oven to 425°F (220°C). Grease an 8x8-inch or 9x9-inch baking dish with butter.
In a large pot or Dutch oven, melt 2 tablespoons of butter over medium heat. Add the minced onion and sauté for 5 minutes until soft and translucent.
Add the minced garlic, salt, and black pepper. Cook for another 30 seconds until fragrant.
Stir in the chicken stock, heavy cream, and sliced russet potatoes. Bring the liquid to a gentle simmer, being careful not to let it boil.
Cover the pot and reduce the heat to medium-low. Cook for 15 to 20 minutes, or until the potatoes are just tender.
Carefully transfer the potato and cream mixture into the prepared baking dish, spreading it out evenly.
Sprinkle the grated cheddar, provolone, and parmesan cheeses evenly over the top of the potatoes.
Bake uncovered in the preheated oven for 10 to 15 minutes, or until the cheese is melted, bubbly, and golden brown on top.
Remove from the oven and let rest for 5 to 10 minutes. Garnish with fresh chopped parsley before serving.