Saute onion and garlic in the oil until soft, 3 to 4 minutes.
Add bay leaf, broccoli, potato, and stock. Simmer gently, covered for about 10 minutes, until the broccoli is tender, but still bright green. Remove bay leaf and let cool.
Puree in a blender (not totally smooth). Season to taste, add lemon juice and return to pan to reheat if necessary. Add a dollop of yogurt just before serving.
First Ladies Cookbook: Favorite Recipes of all the Presidents of the United States by Robert H. Doherty