From http://www.closetcooking.com/2012/03/corned-beef-grilled-cheese-sandwich.html

Ingredients:

• 1-2 yellow onions, sliced up
• 1 Guinness
• 1 tablespoon olive oil (plus some butter, cause it makes everything delicious)

Directions: Heat up oil and butter. Add in onions. Cook 5-7 minutes til just soft. Add 1/4c or so of Guinness. Cover and boil down for about 15 minutes. Uncover, stir, and repeat 3 more times. Enjoy.