For the mayonnaise: In a food processor, combine the egg, lime juice, serrano and cilantro, and pulse until blended. With the machine running, slowly drizzle in the oil (the egg and oil will emulsify and become creamy). Let the processor run for another 20 seconds to ensure that everything is well incorporated. If chiles or cilantro fly up onto the processor bowl, stop the machine to scrape down the sides. Taste and season with salt, usually about 1/2 teaspoon. Scrape the mayonnaise into a bowl and set aside.
In a large (10-inch) skillet set over medium heat, cook the bacon, stirring occasionally, until crispy, about 10 minutes. Remove the bacon and drain on a paper towel. Spread some mayo onto each tortilla, top with a gem lettuce leaf, a portion of the tomatoes and a couple of avocado slices. Sprinkle each taco with salt and a tablespoon of bacon, garnish with a little more mayo and cilantro leaves and serve to your guests right away.