This is such an easy dish. It takes some time to put together, though — primarily because of caramelizing the onions. Caramelizing onions, however, is a great make-ahead step. You could caramelize the onions on a Sunday night, or even prep the entire dish ahead of time and then bake it after you get home from work.
It pretty much goes without saying that this dish makes fantastic lunch leftovers, too. It would also freeze magnificently.