Peel and coarsely grate the carrots. Put to one side. Finely chop the leaves from the sprigs of parsley. Finely grate the zest from the lemon.
Measure out 1 tbsp of the butter and pop it into a bowl. Mash the flour into the butter with a fork to make a roux mixture. Pop to one side.
Measure out 400ml of the milk. Pour it into a pan. Bring it to the boil – keep stirring as it heats up to stop it catching and burning. When the milk is just boiling, add the butter and flour.
Whisk constantly for about 8-10 mins till the sauce starts to thicken. When your whisk leaves trails in the sauce, take it off the heat. Season. Stir in most of the parsley, all the lemon zest and the spinach.
Heat your oven to 180°C/Fan 160°C/Gas 4. Chop the whiting fillets into bite-size chunks. Pop the fish in an ovenproof dish. Stir in the spinach sauce.
Mix the grated carrots with the remaining chopped parsley. Spread the carrot thatch over the top of the pie.
Bake the pie for 20-25 mins till it's bubbling and the carrot thatched is a little charred (they'll burn faster than potatoes would, so keep your eye on the pie). Serve straight away.