Preheat your oven to 400°F. Place the sliced pork belly on a baking sheet and roast for about 15-20 minutes, or until the edges are crispy and golden brown. Keep an eye on it to prevent burning.
While the pork belly is roasting, prepare the baguette. Cut it in half lengthwise, leaving one side attached to create a hinge. Lightly toast the inside of the baguette in the oven for about 5 minutes until warm and slightly crispy.
In a small bowl, mix the mayonnaise with soy sauce and sriracha (if using) to create a spicy mayo.
Once the pork belly is done, assemble the banh mi. Spread the spicy mayo on both sides of the baguette.
Layer the crispy pork belly slices on the bottom half of the baguette.
Top the pork belly with pickled carrots and daikon, cucumber slices, jalapeño (if using), and fresh cilantro.
Season with salt and pepper to taste, then close the sandwich and press down gently.
Slice the banh mi into manageable pieces and serve immediately.