Slice onion into slices.
Heat butter and olive oil in a skillet.
Add onion and stir frequently.
You are basically caramelizing the onions - but this takes forever so I usually cook them for about 25-30 minutes and add a little sugar to help caramelize them.
Slice Hawaiian rolls in half to make the sandwiches.
Add beef tips, blue cheese, and onions.
If you don't like blue cheese you can make this with sharp cheddar and dijon mustard or your favorite cheese and a spicy mayo. Any way you make these sandwiches they are delicious.
I serve them just like this but you can put the sandwiches on a pan and toast them for 3 minutes in the oven if you want to get fancy.