Melt butter in a medium size skillet and sauté onion. Add curry, cumin and season with salt and pepper. Remove onions.Cut zucchini (unpeeled) in slices and sauté, in batches, in the same butter as the onions. Cover and cook for 20 minutes. Return onion to skillet.
Add 2 cups of hot water, bring to a boil for 5 minutes and blend.Add rest of water (necessary amount) and strain. Correct seasoning.Serve with bread croutons.