Preheat oven to 400°F. Butter a 9 x 13 baking dish. Melt butter in a large sauce pan over medium-high heat. Add onion, and sauté for 4-5 minutes until soft and translucent. Add garlic and sauté for an additional 1-2 minutes until fragrant. Stir in the flour until it is evenly combined, and cook for 1 more minute. Gradually pour in the stock, and whisk until combined. Add in the milk, salt, pepper, and 1 teaspoon thyme, and whisk until combined. Continue cooking for an additional 1-2 minutes until the sauce just barely begins to simmer around the edges of the pan and thickens. Then remove from heat and set aside.
Spread half of the sliced potatoes in an even layer on the bottom of the pan. Top evenly with half of the cream sauce. Sprinkle with 1/2 cup of the shredded Parmesan cheese. Top evenly with the remaining sliced potatoes, the other half of the cream sauce, and the remaining 1/c cup of cheddar cheese.
Cover the pan with a lid or foil and bake for 30 minutes. Remove the foil and bake uncovered for 25-30 minutes, or until the potatoes are cooked through.
When the potatoes are tender and if you want the dish a little more brown on top, turn the broiler on, watching at all times and brown just a little. Sprinkle with fresh thyme. Enjoy!