Melt the butter in a skillet.
Add the onion and sweat for a few minutes.
Add the garlic and cook for half a minute.
Throw in the spinach leaves and sprinkle with salt and pepper. Stir until wilted. Cover the pan and cook over low heat for about three minutes (the spinach leaves have enough water in them to ensure that the mixture does not dry up at this stage).
Pour in the yogurt. Taste. Add more salt and pepper, if needed.
When the sauce starts to simmer, toss in the cooked pasta.
For vegetarians, top with crumbled tofu skin before serving.
For omnivores, sprinkle with crumbled crisp bacon.