Place onion, serrano, 1 teaspoon salt, and lime zest on cutting board and chop until very finely minced. Transfer onion mixture to medium bowl and stir in 1 1/2 tablespoons lime juice. Add avocados and, using sturdy whisk, mash and stir mixture until well combined with some 1/4- to 1/2-inch chunks of avocado remaining. Stir in tomato and cilantro. Season with salt and up to additional 1 1/2 teaspoons lime juice to taste. Serve.