Peel and finely chop the onion. Trim and rinse the celery. Finely slice. Rinse the baby leaf spinach in a colander. Pat dry with kitchen paper. Pop to one side for later.
Heat 1 tbsp olive oil in a large pan with a lid. Add the onion and celery. Season with a little salt and pepper. Cook over a medium heat for 5 mins. Stir occasionally till softened and glossy.
Fill the kettle and boil it. Peel the potatoes. Chop into chunks around 2 cm across.
Add the potatoes to the pan and stir fry for a min. Stir in the milk with 200ml hot water from the kettle. Cover. Simmer gently for 15 mins till the potato is tender.
Remove the pan from the heat. Carefully pour half the chowder into a bowl or jug. Use a hand blender to blitz. Pour back into the pan with the rest of the chowder.
Return to the heat and add the fish pie mix. Cover and simmer for 5-8 mins till the fish just flakes. Stir in the spinach. Cover for 1 min till everything is cooked through and the spinach is wilted.
Taste the chowder. Season with some salt and pepper if you think it needs it. Serve in large warm bowls.