Preheat the oven to 200C/Gas 6. Boil the spinach for 2-3 minutes, drain thoroughly, and finely chop. Season well.
Lightly spray a medium, shallow ovenproof dish with oil. Spread the spinach mixture in an even layer in the bottom of the dish. Place the haddock fillets on the spinach in a single layer and season well. Top with the sliced tomatoes and place in the oven for 8-10 minutes.
Meanwhile, put the yoghurt, herbs, egg yolks, cheese, and nutmeg in a small bowl, mix together, and season well.
Preheat the grill until medium hot. Remove the fish from the oven and spoon the sauce evenly over the top. Place under the grill for 4-5 minutes, or until the sauce is golden and bubbling and the fish has cooked through. Serve immediately.