Peel the onions and slice thinly into rings.Add the butter and oil to a heavy-based frying pan.Set over a medium heat until melted.

Add the onions to the pan and cook,stirring occasionally,for about 5 minutes,or until light golden and softened.

Add the Beef stock to the pan.Mix thoroughly,bring to a simmer and reduce the heat.Cover and cook over a gentle heat for about 30 minutes.

Preheat the oven to 230 oC/450 oF/Gas 8.Meanwhile,toast the bread until light golden.Put 6 small gratin dishes on a baking tray (in my own Words might be possible too make a whole One too serve 2 very hungry People).

Season the onion mixture with salt and pepper.Divide the mixture between the gratin dishes with a ladle.Top each dish with a slice of the toasted bread and sprinkle over the grated cheese.

Transfer the gratin dishes to the preheated oven and cook for about 5 minutes.Turn on the grill and re-position the dishes underneath.Cook until the tops are golden brown. Serve.