Baked Cod with Mustard Cheese Sauce

Cod is one of those fish that forgives a lot. It has big, tender flakes and a mild taste, so it works well under a sauce that has some bite to it. This one is a simple white sauce, the sort you might learn first on a cold afternoon, with grated hard cheese and a spoonful of English mustard stirred through. The mustard matters more than you'd think. It doesn't make the sauce taste of mustard so much as sharpen the cheese and wake up the fish.

A fact worth knowing about cod: it's a lean fish, with most of its fat in the liver rather than in the flesh. That's why it bakes up moist and light without any help, and why a sauce like this is a pleasant addition rather than a rescue. Because the fish is lean, the one thing to watch is timing. Cod goes from translucent to flaky very quickly, so check it at the 15 minute mark by pressing a knife into the thickest part. If it still looks glassy in the middle, give it a few more minutes and check again.

A small tip for the sauce itself. When you add the milk, pour it in slowly and keep whisking, especially at the start, when the flour and fat are still forming a paste. If you get a few lumps, don't panic. Keep whisking over the heat for another minute and they will usually smooth out. Take the pan off the heat before you add the cheese, then stir it in gently so it melts rather than turning stringy. Chopped herbs go in last, so they stay green and bright.

If you have leftovers, let the fish cool and keep it in a sealed container in the fridge for a day. It's good cold, flaked into a green salad with a squeeze of lemon, or warmed gently in a low oven with a splash of milk on top. Cod doesn't reheat well in the microwave, so go slowly if you do.

For serving, I like something green and simple next to it. Steamed green beans, a pile of peas or some wilted spinach all work well, and a few boiled new potatoes help soak up any sauce left on the tray. If you want something lighter, a bowl of Silky Asparagus Soup makes a gentle starter in spring. For a heartier supper, a bowl of Creamy Coconut Fish Chowder would turn this into a proper feast, and a little Smooth Roasted Vegetable Soup on the side works well for a cold evening.