When the butter in the pan has foamed up well and is settling down, add the onion and stir well for a minute. Add the garlic and continue stirring and cooking on medium high heat for another 2 or 3 minutes until the onion and garlic is soft, fragrant and translucent but not browning. Turn the heat down if necessary to keep the onion from browning. Add the additional butter to the onions, and after it has fully melted whisk in the flour and cook for another 2 minutes, stirring the entire mixture constantly. Add the finely chopped greens and cook until the liquid is released from the greens. If you're using the optional egg yolk, whisk it into the cream now. Add the cream, allspice and nutmeg and continue cooking until the liquid is reduced by about half. Salt and pepper to taste. Serve at once.