In a large pot, bring the stock to a boil.
Chop the green chile (if using) and add to stock.
In a large skillet, saute onions and garlic in the oil till golden. Add to the stock pot when done.
Cook cabbage in the skillet till tender. Won't take long. Add that to the pot.
Cook spinach in the skillet till wilted. Will be fast! Add that to the pot.
Bring everything to a boil and boil for a few minutes.
Remove from heat.
Puree everything right in the pot with a hand-held blender. (Can also use regular blender, of course.)
Stir in yoghurt (or see variations below).
Reheat before serving, but do not bring to a full boil.
Optional: If you didn't include green chile, add a little balsamic vinegar. A little adds a nice depth to the soup. Be careful not to overdo.
Optional: Chop fresh herbs and put on top of soup. Or, you can put whole leaves of herbs on the table and scissors and let people add their own. (Basil and/or tarragon are highly recommended.)
Serve with a dollup of yoghurt on top.