Slice up the bacon/pancetta/guanciale. The more fat it has, the better it would be.
On medium heat, render the fat out.
Increase the heat. Mix in the diced up onion and saute until the onion gets a glassy / translucent look for about 5 minutes. Add more fat or oil if neccessary.
Lower the heat to medium to low and slowly caramelize the onions for about 10 -15 minutes.
Meanwhile the onions are getting fried, cut the cabbage up.
Increase the heat to high then mix in the diced up cabbage with the onion and bacon.
Keep stirring the cabbage to prevent it to burn down until it collapses and fry through for about 5 - 10 minutes.