Patty Melts

I've noticed Patty Melts on menus for years, but didn't really know what they were.  The Pioneer Woman had a recipe on her blog, so I gave them a try. They are kind of a cross between a hamburger and grilled cheese sandwich.  They are yummy and gooey, but I'll be honest:  the best parts of a hamburger to me are the tomato and lettuce and pickle on top.  A patty melt doesn't have that fresh stuff, so that's the downside.  But I don't see why you couldn't just add those at the end?  Why not?  I did add mushrooms in with the caramalized onion, so I guess I already cheated.  I don't see how a little more cheating can hurt anything!
This recipe makes 4 sandwiches.
Patty Melts
1 stick butter
1 large onion, sliced
8 oz. mushrooms, sliced, optional
1 1/2 lbs. lean ground beef
Montreal Steak Seasoning or Salt and Pepper, to taste
Several dashes Worcestershire Sauce
8 slices Swiss cheese
8 slices bread

Melt about 2 Tablespoons butter in a medium sized skillet over medium heat.  Cook the onions slowly until caramelized.  It will take about 20 minutes.  The onions should be golden and soft.  Add the mushrooms and cook until browned.
In a bowl, mix the beef with the seasonings.  Divide the meat into four patties.  Melt 2 Tablespoons butter in another skillet.  Over medium heat, cook the patties until done.
Then assemble the sandwiches:  Bread, cheese slice, patty, 1/4 onions, another slice of cheese, another slice of bread.  On a griddle or skillet, melt another 2 T. butter and grill sandwiches on one side over medium heat until golden brown.  Remove them and melt the other 2 T. butter.  Grill the sandwiches on the uncooked side.  It should look like a grilled cheese with lots of good stuff in the middle.  Like I said, next time I'm adding a tomato.