Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.

Prepare the Cream Mixture: In a mixing bowl, combine the heavy cream, minced garlic, dried thyme, salt, black pepper, and cayenne pepper (if using). Mix well.

Layer the Potatoes: Arrange half of the sliced potatoes in the bottom of the prepared baking dish. Sprinkle half of the chopped onion, half of the crumbled bacon, half of the cheddar cheese, and half of the Parmesan cheese over the potatoes.

Repeat Layers: Add the remaining potatoes in another layer, followed by the rest of the onions, bacon, cheddar, and Parmesan cheeses.

Pour Cream Mixture: Carefully pour the cream mixture evenly over the layered potatoes, ensuring that the potatoes are mostly submerged.

Bake: Cover the baking dish with aluminum foil and bake in the preheated oven for 45 minutes. After 45 minutes, remove the foil and bake for an additional 15-20 minutes, or until the potatoes are tender and the top is golden and bubbly.

Cool and Serve: Allow the scalloped potatoes to cool for a few minutes before serving. Garnish with chopped chives or parsley if desired.