Possibly defrost the fish. For the crust, wash thyme, shake dry and chop coarsely. Crumble toast finely between your hands. Knead with butter and thyme. Season with salt and pepper.
Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Wash fish, dab dry, cut into 2 pieces. Season with salt and pepper. Place in a greased ovenproof dish. Spread the toasting mixture over the fish.
Wash and halve the tomatoes and press them lightly into the crust. Bake in a hot oven for about 20 minutes.
Peel and finely chop the onion. Bring the frozen spinach, onion and 7-8 tablespoons of water to the boil, cover and defrost over medium heat for 10-12 minutes. Simmer open for 4-5 minutes. Add cream to spinach and simmer for 3-4 minutes.
Season with salt, pepper and nutmeg.
Serve the fish with the spinach. Garnish with lemon wedges. Serve with brown bread or boiled potatoes.