Things we changed:
1) The biggest change is obviously that we used kale instead of swiss chard
2) We used some butternut squash in addition to sweet potato.
3) We supplemented some of the gruyere with comte that I found at the back of our cheese drawer, and since I am particularly fond of melted cheese, we also added a bit of extra Italian blend cheese on top.
I have to say that this was outstanding. Although my boyfriend loves the bitterness in greens, I'm less of a fan, and sometimes sweet potatoes can be, well, too sweet. Here the balance between the kale and the sweet potato was amazing - not too sweet, not too bitter, just perfect.
Kale & sweet potato gratin: sweet potato, butternut squash, kale, onion, garlic, heavy cream, nutmeg, parsley, thyme, gruyere, comte & Italian blend cheese