Heat the olive oil in a large heavy based pot over medium heat. Add the meat in batches, cooking until just browned.
Remove meat to a plate and in the same pot cook the onion, stirring, until it is softened but not brown.
Put the meat back in the pot and add just enough hot water to cover, salt and pepper. Cover the pot and bring to a simmer. Cook for 1 hour or until the meat is tender.
When the meat has almost cooked add spring onions, dill and your chosen greens. Cook until the greens are tender.
Make the Avgolemono: in a heat proof bowl, beat the egg yolks gradually adding the lemon juice until it is completely incorporated. Mix in the cornstarch, which has been first diluted with a little lemon juice to make a slurry. Then gradually add a few tablespoons of hot stock from the pot to the egg and lemon mixture, whisking rapidly as you add it. This prevents curdling. Once that is done you can add it gradually back into the pot with the fricassee, a little at a time, swirling it through by shaking the pot. Allow the sauce to simmer for a few minutes, but do not boil rapidly - otherwise it will split. Serve hot.