Preheat the oven to 200ºC, fan 180ºC, gas 6. In a large saucepan, melt the butter, then add the onion, leeks and garlic. Gently cook for 5 minutes, until softened.
Meanwhile, bring a pan of water to the boil, add the broccoli and cook for 2 minutes. Drain and rinse under cold water to stop the cooking process, then set aside.
Add the flour to the softened leek mixture, stir, and cook for a further minute. Slowly add the milk, stirring the entire time. When all the milk has been added, continue to stir until the sauce has thickened. Simmer for 2-3 minutes.
Stir in the crème fraîche. Next, add the cooked and drained broccoli, the spinach and the salmon and cod pieces. Mix until well combined, then transfer the mixture to a 2-litre pie dish.
Layer the potato slices on top, overlapping slightly and making sure there are no gaps. Brush with the oil, then bake for 40 minutes.
About 5 minutes before the end of the pie cooking time, add the petits pois to a pan of boiling water and simmer for 3 minutes. Drain and serve with the fish pie.