Begin by slicing the pork tenderloin very thin. Or better yet, ask your butcher to do this for you! They can do it in about 2 minutes and it will save you tons of time. And they will get it much thinner than you can get it at home.
In a blender or food processor, add the shallot, garlic, sugar, honey, fish sauce, sesame oil, salt and pepper. Pulse until combined. It's okay if there are chunks of onion or garlic. You can also just chop the shallot and garlic by hand and mix it all together in a bowl, I'm just lazy.
Add the marinade and thinly sliced pork to a medium bowl, cover, and refrigerate for at least an hour, or up to 24 hours.
Make the picked veggies: Chop the carrots and daikon into matchsticks. See photos. You want them to be about the length of a matchstick and the thickness of your average chop stick. It doesn't have to be perfect! Don't stress about it. Just chop them up thin-ish and your sandwich is going to taste amazing.
Add the chopped veggies to a large bowl and sprinkle with 1 and 1/2 tablespoons salt and 1 and 1/2 tablespoons sugar. Use your hands to massage the sugar and salt into the veggies. We're doing this to get the carrots and daikon to release water, so that they will be flexible but still crunchy. After 15 minutes of sitting in the salt and sugar, you should be able to bend a carrot without it snapping (see photo).
Pour the veggies into a colander and rinse them under water until all the salt and sugar is washed off.
Rinse out the bowl and make the brine: combine 1/2 sugar and 1 cup warm water, and stir to dissolve. Add the vinegar and a few shakes of salt. Stir it all together.
Add the rinsed carrots and daikon to the vinegar mixture, make sure all the veggies are covered, and stick it in the fridge for 45 minutes. (They will keep for months in the fridge!)
When the pork has marinated, heat a large skillet over medium high heat. Add about a tablespoon of oil to the pan. Make sure it is shimmering.
Add the pork strips in batches. If you add it all at once, it will not brown. Make sure there is about an inch of space in between each piece of pork.
Sear the pork for about 1-2 minutes, then flip and sear for another minute or so. If you sliced the pork yourself, you will probably have to cook it longer because it will be thicker. Adjust the heat as necessary, you want to keep it as high as possible without the pan scorching.
Continue searing until all the pork is cooked. Add more oil as necessary. Set aside on a plate, or put it in the oven on a pan on the "keep warm" setting (170 degrees F).
In a small bowl, combine 1 cup mayonnaise and 1 tablespoon Sriracha (more or less Sriracha to taste).
To assemble: Slice a roll in half lengthwise. Spread both sides with Sriracha mayo.
Top one side with pork, sliced cucumber, sliced jalapeno, lots of pickled carrots and daikon, and fistfuls of cilantro (you can chop it if you want, I just tore it from the top of the bunch.)
Top with the other half of the roll and eat! Serve with more Sriracha mayo if you want.