Put oil in large heavy soup pot and turn heat to medium.
Add onions and garlic to the hot oil and cook till golden, stirring frequently.
Add cabbage to the pot and cook till tender. Won't take long.
Add spinach to the pot and cook till wilted. Will be fast.
Add chile.
While onions, etc. are cooking, if you are using BTB, stir it into a small amount of hot water to dissolve. Stir this into the rest of the water to make 3 cups of stock.
Add stock to pot and bring to a boil. 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).
If not serving right away, 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 or sour cream on top.