The premise behind the French method of cooking en cocotte (or casserole roasting) is to slow down the cooking process in order to concentrate flavor.
Fish cooked for an extended period of time usually winds up dry, but a combination of low oven temperature, moist-heat environment, and the right cut of fish allows it to remain juicy and tender.
We found that meaty swordfish steaks were particularly well suited to cooking en cocotte. The fresh Mediterranean flavors of mint, parsley, lemon, and garlic easily combined with sliced cucumber to make an insulating layer on which to cook the fish; we then turned the cucumber mixture into a complementary flavorful topping for serving.
It is important to choose steaks that are similar in size and thickness to ensure that each piece will cook at the same rate. If swordfish isn’t available, you can substitute halibut.