A versatile vegetable, I tasted a chorizo onion in Paris at Passage 53, a two star Michelin and I was impressed by the simplicity: http://mesubim.com/2014/11/03/onion-chorizo-passage-53-restaurant/
This dish is so easy to do and by boiling the onion, you soften it getting it ready for infusion of other flavors. In my case I simply colored and flavored it with a homemade tare I made.
If you use butter it adds some fat to the onion and when boiling you must change the water or the onion taste remains too dominant. The chef at Passage 53 uses butter and adds the chorizo between the onions’ layers.
What I like about this dish is that it takes very little time to do, it looks good and tastes good.