Heat olive oil over medium till hot (not smoking). Sauté onion in the oil till it begins to turn golden, about 10 minutes. Add garlic and continue to sauté for 3 minutes more. Add yams and zucchini, sauté for 2 minutes more. Add chicken or vegetable stock, thyme leaves, cumin, ginger, and cayenne. I like to use ¼ tsp of cayenne, which gives a nice spicy kick to the soup. If you’re sensitive to spice, start with 1/8 tsp and add more to taste, if desired. Bring mixture to a boil, then reduce to a light simmer for 20 minutes.