1) Heat oil over med. heat.
2) Add onions and sauté until tender.
3) Add 2oz Guinness, cover and simmer until liquid is almost evaporated. (repeat three times)
4) Remove from heat.
5) Stir in Worcestershire sauce and mustard.
6) Heat a pan over med. heat.
7) Butter the outside of each slice of bread.
8) Place one slice of bread in pan (butter side down)
9) Sprinkle half of the cheese on one slice of bread.
10) Top with half of the onions, corned beef, other half of the onions, cheese and finally the other slice of bread. (butter side up)
11) Grill until golden brown and the cheese has melted (2-4 minutes each side)