Add butter, heating until butter is slightly foaming
Add onion slices, stirring to coat with oil, then spread in an even layer.
Reduce heat to medium, and cook onions for 10 minutes, stirring occasionally, until brown and caramelized.
Reduce heat to low and allow to sit while you assemble the pudding
Preheat oven to 350° Butter an 8" casserole dish
Put bread cubes in an even layer on a baking sheet and put into the oven for 15 minutes to toast, while you continue with the recipe.
In a medium bowl, combine eggs, beer, cream, cayenne and mustard
Remove onions from heat. In the casserole dish, toss together the toasted bread cubes, 3 cups of the cheese, and onions, spreading evenly.
Pour beer mixture over the bread mixture. With a spoon, make sure all the bread is combined with the liquid
Bake 45 minutes, or until a knife inserted into the center comes out clean, and the top is golden and crisp
Spread remaining 1 cup cheese over the top. Bake 5 minutes more, until cheese is just melted.
Remove from oven and allow to cool 10 minutes before serving. Garnish with parsley.
*Oven toasting bread cubes is a quick way to get "stale" bread and gives a better flavour and texture, too.*This dish will taste better if allowed to cool somewhat to allow the steam to escape and to let it dry out a little bit. It's wonderful cold, too!