Combine the julienned carrot and daikon in a jar or bowl. Whisk together the rice vinegar, warm water, sugar, and salt until dissolved, then pour over the vegetables. Let pickle for at least 30 minutes, or make up to a week ahead.
Toss the sliced pork with soy sauce, fish sauce, brown sugar, and garlic. Let marinate for 10-15 minutes if time allows.
Heat the oil in a skillet over medium-high heat and cook the pork for 3-5 minutes per side until browned and cooked through. If using leftover chicken, simply warm it through.
Slice open the baguettes and lightly toast if desired. Spread mayonnaise, and pate if using, on one side.
Layer the cooked protein onto the bread, followed by a generous amount of pickled carrot and daikon, cucumber slices, cilantro sprigs, and jalapeno slices.
Drizzle with a little extra soy sauce, close the sandwich, and serve immediately.