Caramelize the onions by cooking them on medium low heat in 3 Tbs butter until deep brown. (this can take awhile, but prep your other ingredients while you're waiting)
Salt and pepper the roast on both sides and place in the slow cooker.
Add soup mix, broth, wine, garlic and caramelized onions.
Cook on low for 8 hours (or longer)
Reserve about 3 cups or so of the juices, let it stand in a bowl for 10 minutes, skim off fat.
Shred the beef with 2 forks, it should be very tender. Return it back to the slow cooker.
Slice the rolls and butter them, place under the broiler for a few minutes until golden.
Pile some beef on the bottom part of the roll and top with a slice of cheese, return to the broiler to melt the cheese.
Cover with the top of the bun and serve immediately with au jus for dipping.