Heat oven to 180C/160C fan/gas 4. Place the spinach in a large colander in a sink and carefully and slowly pour over a kettleful of hot water to wilt it. Cool again under the cold tap then squeeze out as much liquid from the spinach as possible. Butter a medium gratin dish. Roughly chop the spinach and scatter evenly over the base of the dish. Season the spinach lightly with salt and pepper and lay the haddock fillets, skinned side down on top. Nestle the tomatoes among the haddock fillets.
Mix all the ingredients for the topping, except the breadcrumbs, in a small bowl and season with lots of pepper and a little salt. Dollop and spread the mixture over the fish and spinach. Scatter everything with the breadcrumbs and bake for 30 mins until bubbling and golden. If you like things really grilled finish for a few mins under a hot grill. Serve straight from the dish.