Preheat the oven to 200°C.
Place salmon and lemon in a tray, cover with water, and bake for 15-20 minutes.
Cool the fish slightly and flake into large pieces.
Blend basil, garlic, pine nuts, and two-thirds of the watercress into a chunky paste, adding oil gradually.
Heat a griddle pan until very hot.
Coat courgettes in oil and seasoning, then griddle in batches for 1-2 minutes until cooked.
Cut the top quarter off the bread loaf and set it aside.
Scoop out the internal bread from the loaf.
Spread pesto onto the bottom of the hollowed loaf.
Layer a quarter of the courgettes and a quarter of the remaining watercress into the loaf.
Repeat layers of salmon, peppers, onions, and pesto until the loaf is full.
Wrap the loaf tightly in cling film and chill for at least one hour or overnight.
Unwrap and cut the loaf into wedges to serve.