Put a large pan of salted water on to boil. Peel and finely chop the onion. Trim the dry ends off the fennel and slice off the thick base. Keep the feathery fennel fronds for later. Halve the fennel, place cut-side down on your chopping board and thinly slice.
Peel and crush or finely chop the garlic. Grate the zest from the lemon (keep the lemon for later). Finely chop the chives.
Heat a deep frying pan for 1 min. Add 1 tbsp olive oil, the onion and the fennel. Season with salt and pepper. Cook gently over a medium heat for 8 mins till softened, stirring now and then.
Add the spaghetti to the pan of boiling water. Simmer for 8 mins till the spaghetti is cooked but not too soft – it should still have some bite.
Stir the garlic and lemon zest into the onion and fennel. Add a pinch or two of chilli flakes (they’re spicy, so add to taste). Cook and stir for 1 min. Take off the heat.
Drain the spaghetti and add it to the onion and fennel. Flake in the salmon, discarding the skin. Add the chives. Halve the lemon and squeeze in the juice from 1 half. Toss to mix, then cover and leave to sit for 2 mins to allow the flavours to mingle.
Whisk 1 tbsp balsamic vinegar with 1 tbsp olive oil and some salt and pepper. Toss with the salad leaves. Taste the pasta and add more salt and pepper if you think it needs it. Heap onto warm plates or bowls, top with any saved fennel fronds, and serve with the salad and wedges of lemon for squeezing.