The gratin dish has quickly become our favorite thing ever. The best part is that it's HUGE and thus holds enough content to create actual leftovers. We have been making and eating every gratin in the land, most notably chard and cauliflower.
On this particular occasion, we were making pasta puttanesca--i.e. with a simple tomato and olive sauce--for dinner. This is fine and all, but it's fairly plain. So, to dress it up, we threw the finished & sauced pasta in the gratin dish, scattered a mixture of breadcrumbs, cheese, and herbs over the top, and baked it at 375F for about a half hour.
Clearly, the crispy business on top is the best part of any gratin. In fact, you can use it to transform practically anything reasonably bakeable--such as the aforementioned pasta--into a pan of glorious deliciousness. Go forth!
Crispy crunchy gratin topping
breadcrumbs (or just finely chopped bread)
various seasonings, herbs, or vegetables
Essentially, we just want to finely chop (or grate, or blitz in ye olde food processor) everything, mix it all together, then spread it over a pan of whatever it is you want to gratinée.
The proportions here are almost totally up to you. I like to start with a more or less equal ratio of breadcrumbs to cheese, but it's also totally fine to go for lots of breadcrumbs and only the tiniest bit of cheese, or a lot of cheese and only a few breadcrumbs. Then you can add a whole lot of different herbs or seasonings, or you can leave them out entirely. It's all about using (or using up) whatever you have on hand.
So. Mix your desired amount of breadcrumbs with your desired amount of grated cheese. I usually make my own breadcrumbs by chopping up any stale (or not stale) ends of bread I have lying around. You can use nearly any kind of cheese you want. Obvious grating cheeses like parmesan and romano work well, but so do gruyere, emmenthaler, white cheddar, and gouda. If you want to crumble in some blue cheese or feta instead of grating anything, go for it. It all depends on what you have and the particular flavor combination you're going for.
Mix a drizzle of olive oil or melted butter with the crumbs and cheese. Use just enough oil to moisten everything slightly. Season with a tiny bit of salt (be careful here, since cheese is salty) and several good grinds of pepper.
At this point, you can chop and add whatever else you like to season the gratin mixture. We decided to add some fresh parsley, a handful of green olives, and a couple cloves of garlic. Other things that might be good: chopped shallot or green onion, any other herbs you like, some paprika or hot pepper flake, a spoonful of pesto or tapenade, or some roasted red pepper or sun-dried tomatoes. You could do a red pepper and shallot crumb for a pan of baked macaroni and cheese, a black olive and basil crumb for cauliflower gratin, a feta and sun-dried tomato crumb for a dish of zucchini and eggplant, or a simple cheddar crumb for a dish of scalloped potatoes. Anything goes as long as it sounds good to you.
Now spread your gratin topping lovingly over your dish of food, stick it in the oven (on a cookie sheet if there's any danger of overflow), and bake it. The temperature and timing will vary depending on what you're making, but it's generally reasonable to start out at 350 or 375F and check for progress after about half an hour.
The completed gratin will be beautifully browned, crispy, and fragrant on top, and hot, bubbling and moist underneath. Hooray! Complementary textures for the win!