In a large heavy bottomed pot, set to medium heat; sweat the onion in 1/4 cup of vegetable broth for 2 to 3 minutes until onion is translucent.
Add corn and garlic and pour in remaining ¼ cup of vegetable broth, cover and let steam until corn is tender, about 3 – 4 minutes.
Uncover, add zucchini and cook for 3-4 minutes, until it begins to soften.
Add tomato and cook for 5 minutes more, or until all the vegetables are tender.
Season to taste, and serve on warm tortillas with avocado slices and salsa.