Wash the greens under a cold tap. Drain them well, then chop roughly and set aside.
Pour the olive oil into a heavy-based pan, add the crushed garlic and the spring onions. Cook for a few minutes until softened but not coloured, then add the chopped greens, a handful at a time.
If the pan starts to overflow, leave the greens for 1-2 minutes to cook down, then add the remainder.
After a few minutes of cooking, gradually add the vinegar and stock - don't add so much stock that the greens become soaked. Add salt (or soy sauce or tamari) and freshly ground black pepper to taste.
When the greens are tender and shiny, remove from the heat and transfer to a serving dish. Serve hot.