Begin by peeling and thinly slicing your potatoes to about 1/8 inch thick. Place sliced potatoes in a bowl of cold water to prevent browning. Drain them in a colander and pat dry with a towel.

In a large skillet, heat a tablespoon of oil or butter over medium heat. Sauté onions until soft, about 5 minutes. Add minced garlic; cook until fragrant, about 1 minute. Add ground beef, breaking it up as it cooks. Season with salt, pepper, smoked paprika, and thyme until fully cooked.

Preheat oven to 375°F (190°C). Grease a large baking dish. Layer half of the sliced potatoes on the bottom, sprinkle with salt and pepper, then add the beef mixture, followed by half of the shredded cheddar cheese. Repeat with remaining potatoes, beef, and cheese.

In a small saucepan, gently heat the milk and heavy cream over low heat until warmed. Pour evenly over layered casserole ingredients.

Cover with aluminum foil; bake in the preheated oven for 45 minutes. Remove foil and bake for an additional 15-20 minutes until golden brown and tender.

Let the casserole rest for about 10 minutes before serving. Garnish with fresh herbs if desired.