Put the pasta in a pan of boiling water and cook to pack instructions. Drain, reserving 150ml of the pasta water.
Meanwhile, heat the oil in a large pan over a medium heat and cook the onion and garlic for 5 minutes until softened. Stir in the crème fraîche, lemon zest and juice, and garden peas. Bring to a gentle simmer, then stir in the pasta along with the reserved cooking water.
Stir through the salmon and half of the chopped parsley. Divide between bowls and serve with the remaining parsley and freshly ground black pepper.