Melt butter in a large pot over medium heat. Add zucchini, onion, parsley and basil and saute until tender, about 5 minutes.
Sprinkle flour, salt and pepper over vegetables. Stir to combine. Gradually whisk in the vegetable broth to make a smooth stock. Add lemon juice and stir once again. Bring to a boil; reduce heat to a simmer and cook, stirring often, for 2 minutes more.
Add tomatoes, corn, and evaporated milk and return to a boil. Reduce heat; cover and simmer for 5 minutes, until corn is tender. If using, stir in fresh greens until just wilted.
Add cheeses and stir until melted. Serve warm and enjoy!