Heat your oven to 200°C/Fan 180°C/Gas 6. Zest the lemon. Trim the courgettes and cut into thick batons. Spread out in a roasting tin and toss with the lemon zest and 1 tsp oil. Season well. Roast for 25 mins.
Crumble the stock cube into a measuring jug. Pour in 750ml boiling water and stir till the stock cube dissolves. Set aside. Peel and finely chop the onions. Scrub and finely chop the potatoes.
Tip the onions and potatoes into a large pan along with 3 tbsp water. Season. Sweat the veg over a medium heat for about 5 mins till the veg start to soften. Pour in the stock. Cover and bring to the boil. Simmer for 10 mins.
Shred the lettuce leaves. Add them to the pan with the roasted courgettes and half the basil leaves. Simmer, uncovered, for 3 mins till the lettuce has wilted.
Use a hand-held blender to blitz everything together to make a smooth soup (or leave it chunky, if you prefer). Squeeze in the juice from half the lemon. Taste and adjust the seasoning if you think it needs it.
Peel and discard the skin from the flaky roast salmon. Flake the flesh into rough chunks.
Spoon the soup into warmed bowls. Scatter the salmon flakes and the rest of the basil on top. Finish with an extra squeeze of lemon juice and some pepper.