Heat your oven to 180°C/Fan 160°C/Gas 4. Scrub the sweet potatoes and chop them into finger-size batons. Put them on a baking tray with ½ tbsp oil and a pinch of salt and pepper. Toss to coat. Roast for 25 mins till starting to soften.
While the sweet potato chips bake, peel and finely chop the onion. Pop it in a bowl. Quarter the cherry tomatoes and add them to the onion. Finely grate in the lemon zest and squeeze in the juice from half the lemon. Finely chop the tarragon leaves and stir them into the bowl. Season with a pinch of salt and pepper. This is your salsa.
When the sweet potatoes have baked for 25 mins, take them out of the oven. Brush the cod fillets with 1 tsp oil each and place them on the baking tray. Return to the oven and bake for 10 mins. The chips should be tender and lightly charred and the cod will be white and flake easily.
Toss the lamb’s lettuce with ½ tbsp oil and a good pinch of salt and pepper.
Serve the cod with the sweet potato chips, the tarragon salsa and the lamb’s lettuce.