Bring a large saucepan of salted water to a rolling boil, add 200g spaghetti or tagliatelle, cook until al dente (8–10 minutes), reserve 100ml pasta water, then drain.
Heat 1 tablespoon olive oil in a frying pan over medium heat, add the finely chopped small onion and sauté for about 5 minutes until softened and translucent.
Reduce heat to low, pour in 200ml crème fraîche, add the zest and juice of half a lemon, stir and warm through gently for 1–2 minutes.
Add the drained pasta to the pan along with 150g torn smoked salmon and most of the finely chopped fresh dill, pour in a splash of the reserved pasta water, and toss until the pasta is coated and the salmon is warmed through.
Season generously with freshly ground black pepper (no additional salt needed).
Divide the pasta between warm bowls, sprinkle over the remaining fresh dill, and serve immediately.