Bring a medium pot of water with the stock cube to the boil and cook the pasta to al dente according to the package instructions. Reserve 1 cup of the cooking water, then drain. Keep the pasta warm in the pot under a lid.
Meanwhile, pat the scallops dry and season them with salt and black pepper.
When the pasta is half cooked, heat half of the oil in a frying pan over medium-high. Add the scallops and sear them for about 2 minutes, then flip them over and cook on the other side until golden brown. Add the lemon juice and a spoonful of the stock - let it bubble and reduce. Transfer them to a plate and keep warm.
Heat the remaining oil in the pan, then sauté the onion for 5 minutes. Add the garlic and stir for a few seconds, then add the wine.
Let it bubble and reduce by half. Add the stock and reduce it until you get a thick sauce. Add the pasta and stir well.
Season to taste and add the chopped parsley and lemon zest.Toss to coat.
Add the scallops and any juice.
Serve immediately with grated Parmesan and/or ground pepper.