Start with some large diameter thin 85/15 beef patties (2 required for each Jucy Lucy). Add salt and pepper. Add sauted mushrooms and onions. Add shredded cheese (this was colby/jack). Seal the innards between the two burgers (seal them good so the cheese stays inside as it melts).
Grilling these on a normal grate is NOT recommended since they are a bit fragile when you first flip them. Try grilling them on a pizza mesh or similarly small-spaced-opening surface.