Grease a large pot with cooking spray and heat over medium heat. Cook shallots, green onions, and garlic 5 minutes or until soft. Mix in zucchini and potato and cook 5 to 8 minutes.
Mix in broth, kale, and tarragon and let boil. Lower heat, place lid of pot, and let simmer 10 to 15 minutes or until vegetables are tender.
Using a food processor or blender, puree mixture until there are no lumps. Pour mixture back into pot and whisk in half-and-half. Soup can be served warm or chilled. When ready to serve mix zucchini and squash slices into soup and dust cayenne pepper over top.