For pickles: Place water, vinegar, sugar and salt in a large pitcher and stir together until liquid dissolves. Mix together daikon and carrot and divide into 2 large jars. Pour vinegar mixture into each jar, until the vegetables are fully submerged. Seal and refrigerate. (these will yield the best results when made 1 day ahead of time)
For beef: Place all ingredients, except beef, into a mixing bowl and whisk together. Add beef and gently fold together until all the beef is marinated. Cover and refrigerate for 30 minutes.
Heat a skillet over medium-high heat and l lightly grease with oil. Add beef and cook for 2 to 3 minutes on each side and remove onto a plate. Set aside.
To assemble: Slice each baguette and smear both cut sides with mayo and pate (if using). Top the bottom half of the baguette with the beef, followed by some sliced cucumbers, jalapenos (if using), pickled daikon and carrot, and cilantro sprigs. Close sandwich with top half of the baguette and gently press together. Repeat until all banh mi’s have been made. Serve.