Cheese And Potato Pie No Pastry

Cheese And Potato Pie No Pastry - 3 lbs of potatoes chopped (or make up to 3 lbs with any chopped root veg) 2 onions chopped in half and very thinly sliced 2 oz grated cheddar cheese tablespoon of margarine or butter (or fat saved from bacon) thyme, salt and pepper.