Cut the ends off the leeks. Cut in half lengthways, then finely slice. Peel and finely chop the onion and garlic.
Melt the butter in a sauté pan over a medium heat. Add the leeks and onions and cook gently for five minutes, until just starting to colour, then add the garlic and cook for 30 more seconds.
Tip in the wine. Allow to reduce by about half.
Turn the heat down to the lowest setting and add the cream. Stir, season with salt and pepper and let it get up to a very gentle simmer.
Meanwhile, bring a large pot of salted water to boil. Tip in the gnocchi and cook for one minute, until starting to soften. Drain, then tip it straight into the creamy sauce. Simmer for another minute before serving.