Heat your oven to 180°C/Fan 160°C/Gas 4. Halve or quarter the potatoes so they’re roughly the same size. Add to a pan and cover with boiling water. Add a pinch of salt.
Bring the potatoes back to the boil. Reduce the heat and simmer for 10-15 mins till tender – a knife tip should be easy to insert into them. Drain and leave to cool slightly.
While the potatoes cook, peel and finely slice the shallots. Zest the lemon. Cut in half and set one half aside. Cut a few slices from the remaining half and juice the rest of that half.
Cut 2 large squares of baking paper, about 35 cm square. Pile a handful of spinach in the centre of each square. Scatter over the shallots. Place the trout fillets on top and season.
Lay the lemon slices on top and pour over the lemon juice. Fold the paper in half and double fold it together, then fold and tuck it underneath the parcel to seal the fish in. Pop in the oven for 15 mins.
Meanwhile, use a vegetable peeler to peel long ribbons from the courgette and the carrot. Trim the leaves off the radishes. Halve or quarter them. Finely chop a handful of the leaves. Place in a bowl.
Squeeze over the juice from the remaining lemon half and add the zest. Season. Roughly chop the dill and add to the vegetables along with the warm potatoes. Add 1 tbsp olive oil. Stir to mix.
Remove the trout parcels from the oven. The fish will be opaque, cooked through and the spinach wilted. Serve with the lemony garden potatoes.