Heat your oven to 200°C/Fan 180°C/Gas 6. Put a roasting tin in the oven to warm up.
Scrub the potatoes and chop them into 1cm chunks. Halve the fennel. Slice it into 8 wedges, picking off any frondy bits and saving them for later. Slice the lemon into 4 wedges.
Take the roasting tin out of the oven. Add ½ tbsp olive oil, the potatoes, fennel and lemon. Season with salt and pepper. Toss to mix. Slide into the oven and roast for 25 mins.
While the potatoes and fennel roast, halve the cherry tomatoes. Peel and thinly slice the garlic clove. Pick the leaves off the rosemary sprigs and roughly chop them.
Take the roasting tin out of the oven. Add the garlic and rosemary and toss to mix. Nestle the cherry tomatoes and lemon wedges in the tray. Add the fish pie mix. Return to the oven and roast for 10 mins. The potatoes should be golden, the fennel tender and the fish opaque – it will flake easily when pressed with a fork.
Add the spinach to the tray and turn everything over a few times to just wilt the spinach. Pile large spoonfuls up in 2 warm bowls, keeping the roast lemon wedges on the side. Garnish with the reserved fennel fronds and serve with the lemon wedges for squeezing.