Heat the olive oil in a large frying pan over medium-low heat, and add the onion. Cook for 6-7 minutes, until softened, adding the garlic for the final minute of cooking time.
Meanwhile, bring a large pot of water to the boil, and cook the pasta for 7-9 minutes, adding the broccoli florets for the final 4 minutes. Drain, reserving about 5 tbsp of the cooking water.
Add the salmon to the pan with the onions and cook over a low heat for 3 minutes, adding the crème fraîche for the final minute.
Add the pasta and broccoli to frying pan along with the reserved pasta water and lemon juice and give everything a stir.
Divide the pasta between four plates, and scatter over the lemon zest and toasted hazelnuts.