In a frying pan over a low heat add the butter and oil and when the butter has melted stir well.
Add the sliced onions, stir well to coat with oil/butter and cook over very low heat for approx 1 hour torning occaisionally until they caramelise.
Remove from the pan and set aside while you cook the sausages.
To Cook the Sausages:
Add the sausages to the pan with a little extra oil if necessary. Cook for 12 minutes turning regularly until browned all over.
Remove the sausages from the pan and carefully slice them lengthways nearly all the way through so they fold open but stay attached.
Return the sausages to the pan cut side down for a further 3 minutes. This browns the centre making sure it is cooked thoroughly.
To Assemble the Sandwich:
Lay two slices of the bread or toast on your board and lay the sausages flat side down on top. Add your red or brown sauce in the channels between the sausage halves.
Spread the onions on top, cover with the remaining bread slices, and cut in half to serve.