Preheat the oven to 200°C, 180°C fan, gas 6 and the grill to medium-high. Toss the sweet potatoes with 1 tablepsoon of the olive oil and the herbes de Provence. Put on a baking tray and bake in the oven for 30 minutes.
Meanwhile, heat the remaining oil in a pan and cook the leeks for 10 minutes, adding the garlic for the last minute.
Make the sauce with the milk to pack instructions. Pour into the pan with the leeks, bring to the boil and cook for 2 minutes before adding in the peas, spinach, parsley and fish pie mix. Pour the mixture into a 1.5-litre ovenproof dish and sprinkle over the cheese.
Put under the grill for 10-15 minutes until golden and bubbling and the fish is cooked through. Serve with the sweet potato wedges.