Method:

Preheat the oven to 180C and prepare an oven safe casserole.
Heat a medium sized pot and add the butter. Once the butter has melted, add the onions and garlic, and saute on medium heat until soft. Add the flour and stir to combine.
Add the milk, chicken stock, thyme and a few pinches of salt. Stir and bring to a boil. Continue stirrin and cook for a few minutes until slightly thickened.
Remove the pot from the heat and add all the cheese and combine.
Add the 1/3 of the potatoes to the casserole and spread 1/3 of the cheese sauce onto the process, repeat the process two more times. There should be three alternate layers of potatoes and cheese sauce.
Cover the casserole with foil, bake for 30 minutes. Uncover the casserole and bake for another 2-30 minutes until golden brown.
Cool for 10 minutes, top with dollops of tangy mayonnaise. Serve and enjoy.