In a rimmed saucer combine the lime zest and salt and mix well. Run the lime slice around the rim of a stemmed, wide mouthed glass to moisten it, then turn the glass upside down and press the rim in the salt mixture. Set glass aside.
In a cocktail shaker combine all remaining ingredients and shake well to chill. Strain into the prepared glass and serve immediately.
If you prefer frozen margheritas then use a blender instead of a cocktail shaker.
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