Homemade pork tenderloin, avocado, mayo and tomato sandwich

This is one of my favorite sandwich, an absolute classic in Chile where is know as "Lomito italiano" because mimics the colors of the Italian flag, include: thinly sliced ​​pork loin, avocado, ripe tomatoes, mayo inside an hamburger bun. It can be lunch or dinner. Is one of the things I always have to have when I go to Chile.
 
Homemade pork tenderloin, avocado, mayo and tomato sandwich
for 4 generous sandwiches

Ingredients: 
for the tenderloin,

•     1 pork tenderloin 500 grams or a pound
•     1 onion, chopped 
•     1 tablespoon of Dijon mustard
•     1 tablespoon oil
•     2 cups boiling water
•     salt, pepper and paprika to taste

for homemade mayonnaise,

•     2 egg yolks or 1 whole egg
•     2 teaspoons lemon juice
•     1 tablespoon white vinegar
•     1 / 2 teaspoon fine salt
•     2 pinches of sugar
•     1 cup vegetable oil

for the sandwiches,

•     4 hamburger buns, lightly toasted
•     2 tomatoes, sliced
•     2 large or 4 medium avocados, mashed

Preparation:

1.     Prepare the tenderloins in advance, hopefully the day before. In a medium saucepan heat the oil over medium-high heat and brown the tenderloin on all sides, leaving about 2 minutes per side, add salt, pepper, paprika, onion, mustard and 2 cups of boiling water and cook over low heat 2 hours, until the loin is tender, adding more water if necessary, it should not dry. Let cool, remove the loin and cut into thin slices, do so against the direction of fibers grow and return meat to pan with the juices and onion.
2.     For the mayonnaise: Place egg in a bowl, add the salt and sugar and mix with the immersion blender, add half the lemon juice and vinegar and then without turning off the blender begin adding the oil in a thin drop, when half the cup of oil is add, add the other half of vinegar and lemon juice. Finish with the rest of the oil. Keep refrigerated for up to 2 weeks.
3.     To assemble the sandwiches lightly toast the bread open, put a layer of mayo in both halves then add the tenderloin, avocado and tomato, cover with half of bread and eat immediately.