Place the potatoes in a pan of water with a pinch of salt and bring to the boil over a medium heat. Once boiling, reduce the heat to a simmer and cook for about 8 minutes, until tender. Drain and leave them to steam dry for 2–3 minutes.
Mash the potatoes, then add the tuna, breaking it up with a fork. Squeeze in the lemon juice to taste, stir in the chives and season with a pinch of salt and plenty of pepper. Add the beaten egg, then use your hands to bring everything together. Divide the mixture into eight even-sized balls and shape into fish cakes. Chill for at least 15 minutes or up to 24 hours is ideal.
Heat the olive oil in a large non-stick frying pan set over a medium heat. Dust the chilled fish cakes lightly in the flour, shaking off any excess, then add to the hot oil and cook for 3–4 minutes on each side, until golden brown.
Meanwhile, put the courgettes and spinach in a steamer. Cover and cook for 1-2 minutes until the spinach has just wilted and the courgettes are tender. Add lemon juice to season and season with a pinch of salt and plenty of pepper.
Arrange the tuna fishcakes on warmed plates with the steamed spinach and baby courgettes to serve.