Heat the oil in a large pan and fry the onions on a medium heat for 5 minutes.
Push the onions to the outer edge and add the diced bacon and continue frying until bacon is cooked and onions softened (flipping occasionally with a spatula).
Stir in the mushrooms and garlic and cook through (about 5 minutes).
Stir in the flour and cook for another minute.
Add the stock, milk, sweetcorn, spinach, potato slices and ground pepper.
Bring to the boil, then lower to a simmer for 10 minutes.
Add the haddock or frozen fish pie mix and continue to simmer for another 10 minutes (or until the fish is cooked).
Taste and adjust seasoning if necessary.
Serve with toast for dunking!