Preheat the oven to 400 degrees F and spray a 9x13 baking dish with cooking spray. Set aside.

Make the cream sauce. Heat a sauce pan to medium heat, add the butter and onion and toss and cook for 3-4 minutes. Add in the garlic and let it cook with the butter and onion for 1-2 minutes.

Whisk in the flour and cook for 1 minute then slowly add the stock and whisk until fully combined. Pour in the milk, salt and pepper. Whisk slowly for 2-3 minutes to allow the sauce to thicken then remove it from the heat.

Assemble. Layer ½ of the potatoes into the bottom of the pan in an even layer with some overlapping. Pour half of the cream sauce over the potatoes, spreading the sauce enough to mostly cover the potatoes. Sprinkle half of the cheddar cheese over the potatoes along with half of the dried thyme. Repeat with one more layer and top with the grated parmesan cheese and the rest of the dried thyme.

Cover the pan with foil and bake for 30 minutes. Remove the foil and bake for 25-30 more minutes until the edges are nice and golden brown and the cheese is melted and bubbly.

Remove, let cool and top with fresh chopped chives or parsley. Serve warm.