Heat your oven to 200°C/Fan 180°C/Gas 6. Place the garlic cloves on a large baking tray or roasting tin and bake in the oven for 10 mins while you prepare the rest of your ingredients.
Drain and rinse the cannellini beans. Set aside. Trim the top and bottom off the leek. Rinse out any grit and chop into 2cm thick slices.
Peel and slice the onions into thick wedges. Cut the tomatoes in half. Take the baking tray out of the oven.
Add the leek, onions and tomatoes to the baking tray with the garlic. Drizzle over 1 tbsp oil. Scatter over 2 tsp fennel seeds, the thyme leaves and a pinch of salt and pepper. Roast for 15 mins.
After 15 mins, carefully pluck the garlic cloves from the tray. They should be soft. Set them aside.
Tip the beans onto the tray. Add the fish pie mix and gently stir to mix. Put the tray back in the oven for another 10 mins till the fish is cooked through – it should flake easily when pressed with a fork.
Squeeze the garlic flesh out of the skins into a bowl. Pour in 2 tbsp red wine vinegar and ½ tbsp oil. Whisk together with seasoning to make a dressing.
Remove the tray from the oven and pour over the garlicky dressing. Divide the fish and veg between 2 warm plates and serve with a handful of spinach.