Shred zucchini and place in a colander set over a bowl. Sprinkle with salt. Let drain for 25-30 minutes.
Pick up the drained zucchini and squeeze out the released juice. Return the zucchini to the colander and rinse under water. Squeeze out the moisture and set aside.
In a medium pot, melt the butter, add onions. Saute onion for 3 minutes.
Add the vegetable broth, cover and simmer for 20 minutes. Remove from the heat and set aside.
Puree the mixture in a blender. Return to the pot.
Add zucchini and mint. Bring to a simmer, cover and cook for 10 minutes.