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 non-stick 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 courgettes in half lengthways, then slice into 1cm chunks. Cut the pepper in half lengthways, scoop out the seeds and slice into 1cm strips.
Give the potatoes a good shake in the tin, then add the courgettes and pepper. Roast together for a further 20 mins till tender and a little charred.
Brush the skin on the cod fillets 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 crush the garlic and mix it into to the lemon with the tarragon. Divide the cod and veg between two warm plates. Spoon over the tarragon gremolata to serve.