Preheat oven to 400˚F. Grease a casserole dish.
Grate 2mm thinly slice potatoes – using a mandolin.
Fry bacon with onions.
Grate three cheeses.
Combine Gruyere, Fontina, Parmesan cheeses and heavy cream. Add fried bacon with onions, roasted garlic, salt/pepper and fresh thyme.
Add sliced potatoes and toss together.
Put in a baking dish, cover with foil and put in the oven for an hour.
After an hour, remove the foil and bake until golden brown.
Sprinkle with chives.