Fruit jelly of any type, been doing it for years.
There's this thing it's the best grilled cheese sandwich. It contains the following: White or Sharp cheddar, parmesan, Muenster, a slice of tomato, and mozzarella sticks. Few grinds of pepper and a small sprinkle of garlic powder and salt. The bread gets a light coating of butter or oil (spray works best) and on the oiled surface a very light, even coating of parmesan for both sides so that it sticks.
Cook parmesan side out in a cast iron skillet or nonstick surface works best for the cheese. Don't try and flip it too soon or the parmesan crust will separate from the bread. It browns a little faster than bread would, so I like to cover the pan with a lid to retain the heat and melt the inside cheese more quickly.
Serve with homemade marinara. Optionally, an overeasy egg added to the mix sends it over the top. Trust me I've been there. It's heaven. It's tricky to get it into the grilled cheese though. Gotta toast the sandwich halves separately and use the broiler on the inside cheese to melt (but not brown) it without the mozz sticks. Then carefully add the mozz sticks and the egg and put it together gently to serve. If that makes any sense...
The Ultimate Grilled Cheese. I mean that's four cheeses and a parmesan crust, there's no other adjective.