Why don't restaurants use croissants for the bun? Maybe some do and I just haven't discovered them, yet. I've been doing this forever, sort of in the "patty melt" style... butter and fry the inside of half the croissant, add cheese, burger, grilled onions, cheese, top with other half of buttered, fried croissant. Lots of calories, yes, but also lots of flaky, buttery taste. Also, it's messy, so eat with a knife and fork.