In a large nonstick skillet, melt the margarine over medium-low heat. Add the garlic and cook, stirring frequently (for 1 minute), or until soft.
Add the zucchini, yellow squash, broth, salt, and pepper. Bring to a simmer over medium heat. Cover and simmer for 6 minutes, stirring occasionally, or until the zucchini and squash are tender. Remove from the heat. Sprinkle with the walnuts and cheese.