Wash your potatoes well and slice them using the mandolin. Not too thin. Slice the onions on the mandolin too if you like. If you don't have a mandolin, slice them with a sharp knife. Layer the potatoes and onions in an oven proof dish and set aside.
Preheat oven to 400 degrees.
Make a roux. In a medium pot, melt the butter over medium heat and add the minced garlic and thyme. Cook for a couple mins. Add the flour to the mix and cook for a couple mins, stirring with a whisk.
Slowly add the chicken broth, heavy cream and light cream to the roux while you whisk. Cook until the sauce coats the back of a spoon well. Remove from the heat.
Add 1 cup of each kind of cheese bit by bit and stir till its all melted.
Add the cajun seasoning and then the salt after you taste it. Some cajun seasoning has a lot of salt in it so you want to be careful not to add too much.
Pour the lot over the potatoes and shake the pan violently and bang it around so that the sauce gets down in between all the cracks in the potatoes. Spread the rest of the cheese all over the potatoes and then sprinkle the chives too. Cover the dish with foil or a lid.
Cook with the foil on for 30 mins and then take the foil off for the last 15 - 20 mins so that the Cheese gets all brown and yummy. They are ready when a knife slips easily into the center of the dish.