Scald the milk in the microwave for about 4-5 minutes until just under the boiling point.

Meanwhile, in a medium saucepan add the butter and flour and cook together for 2 minutes. Slowly pour in the scalded milk whisking constantly.

Cook until the sauce begins to thicken. Add the lemon zest, Dijon mustard, savoury and season with salt and pepper to taste. Add the Parmesan cheese just before pouring onto the fish.

Arrange the fish in the bottom of a greased 9x9 inch baking dish or in individual gratin dishes. Pour the sauce over the fish. Top with the cheddar cheese.

Mix the cracker crumbs with the olive oil and sprinkle over the cheese. (You can add a little extra savory to the crumbs if you like, about ½ tsp). Bake for 45 minutes to an hour at 350 degrees F until bubbling and the top has evenly browned. Individual gratin dishes should take about 25 -30 minutes, just check the center to see that the fish has fully cooked