Let the spinach thaw.
Preheat the oven to 200°C (180°C fan, gas 6). Lay one salmon fillet, skin side down, on a sheet of baking paper. Cut 2 slices from the lemon and set aside. Sprinkle the salmon with some lemon juice and salt.
Wash the herbs and shake dry.
Place the spinach in a bowl and mix in some herbs. Peel and slice the garlic, and add to the spinach with the crumbs. Season with salt and nutmeg.
Peel the onion and cut into rings.
Arrange on the salmon fillet. Add the remaining herbs and season with pepper.
Spread the spinach over the onions.
Cover with the second salmon fillet, place the lemon slices on top and tie it all together with kitchen string.
Brush the fish with oil, wrap in the baking paper and seal well. Bake in the oven for 30 minutes. Serve with salt potatoes.