Heat the oil in a medium pan. Add the garlic and sauté gently for 3 minutes. Add the potato and sauté for a further 2 minutes.
Add the water and milk. Bring to the boil, cover and simmer for 5 minutes or until the potato is tender.
Add the spinach, cod and peas. Stir until the spinach has wilted and the cod and peas are cooked. Transfer to a blender and purée until smooth or mash with a fork if you want a chunkier texture.