For mayonnaise you will need a blender. Add egg, mustard, lime juice, salt and pepper to blender and then use the lid without the middle part to cover. Start the blender on low so the ingredients don't spray out. Blend the ingredients together for maybe 10 seconds, then slowly pour in oil. It is important to ALWAYS do this slowly, from start to finish, never more than a spaghetti thick string of oil AND, it has to be constant. Don't stop pouring until the mayonnaise is ready. If you pour in more than a thin string you risk the mayonnaise to curdle. If this happens you will have to start all over, so be patient and maintain a steady hand. Once the blender can't continue blending because the mayonnaise has become too thick, stop the oil pouring, your mayonnaise is ready.
Heat two pans on the stove over medium heat.
Wash cilantro and shake well to remove excess water.
Cut and peel avocado half.
Add cilantro, avocado half and 1/2 cup mayonnaise (crushed garlic and deseeded Serrano chillies) into food processor and process until smooth. If needed add 1 Tbsp of water to liquify. I usually process the sauce without garlic and chillies first, separate a part of it for the kids, then crush the garlic into the processor and add the deseeded chillies to process again for the adult's salsa.
Salt and pepper salsa to taste.
Put corn tortillas 1-2 at a time in one pan for about 1 minute per side and once hot and soft put in a kitchen towel to keep warm or a plate covered with a pot lid.
In the other pan, heat a dollop of avocado oil and fry shrimps in it for about 30-60 seconds, not longer. This is just to heat them. Salt and pepper shrimps to taste.
Assemble tacos by placing tortilla first, then shrimps, then tomatoes, then salsa.