Heat your oven to 180°C/Fan 160°C/Gas 4. Scrub the potatoes and chop them into bite-size chunks. Put them in a roasting tin with ½ tbsp oil and a pinch of salt and pepper. Toss to coat. Roast for 20 mins till starting to soften.
Cut the courgette in half lengthways, then slice into 1cm chunks. Trim the dry ends off the fennel bulb (saving any fronds for later), then halve it lengthways and slice each half into around 6 wedges. Cut the pepper in half lengthways, scoop out the seeds and slice into chunks to match the courgette.
Give the potatoes a good shake in the tin, then add the courgette, fennel and pepper. Roast together for a further 20 mins till tender and a little charred.
Brush the skin on each cod fillet with ½ tsp oil and season with a little salt. Push the vegetables to one side and add the cod to the tin, skin-side down. Roast for 8-10 mins till the cod is just cooked – it will be white and flake easily when pressed with a fork.
While the cod cooks, zest and juice the lemon into a small bowl. Pick the leaves from the tarragon and roughly chop them.
Peel and finely chop the garlic and mix it into the lemon with the tarragon. Divide the roast cod and veg between 2 warm plates. Spoon over the tarragon gremolata to serve, garnished with the reserved fennel fronds.