In a large pan of boiling water, cook the spaghetti, broccoli and peas together for 5 minutes.
Meanwhile, heat the oil in a pan and cook the onion until softened and turning golden. Add the garlic and cook for a further 1 minute.
Drain the pasta and veg, reserving 4 tbsp cooking water. Return the pasta and veg to the pan, then stir through the smoked salmon, garlic and onion mixture, the crème fraîche, lemon juice, reserved pasta cooking water and the dill, until the pasta is well coated. Season with freshly ground black pepper and sprinkle over the lemon zest to serve.