Preheat oven to 160˚C.
Optional ; Sauté the onions ahead to caramelised them. In a sauté pan, melt the butter and add the onion, thyme and sauté until lightly coloured, about 6-8 minutes. Remove from the heat once coloured. set aside.
In a saucepan, bring to boil the stock, cream and milk. Add the garlic, the rest of the thyme and season with salt and pepper. Once boiled remove from the heat and keep warm.
In a ceramic / gratin dish, layer each thinly sliced vegetables and cooked onion mixture alternating colours and flavours.
Pour over the cream and stock mixture until the vegetables are just covered.
Scatter over the cheese.
Cover loosely with parchment paper and foil.
Place in the oven for 60-80 minutes or until cooked through!
Serve the gratin while hot.