In a large saucepan, melt butter with oil and sauté onions until translucent.

Add the asparagus, spinach, watercress and lettuce and cook, stirring occasionally.

Add half of the broth, cover, and simmer mixture over medium heat for about 15 minutes.

Next, place broth mixture – this is easiest done in batches – in a food processor or blender and pulse to puree.

Once entire mixture is pureed, return to saucepan, add remaining broth and simmer, uncovered for about 10 minutes.

Add salt and pepper to taste and serve hot, garnished with the lemon zest.

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