To prepare marinade, add all of the ingredients to a large bowl and whisk to combine. Reserve 1 cup of the marinade and set it aside. Using a rolling pin or meat tenderizer, beat the chicken until it's about 1/2-inch thick. Place the chicken breasts in the bowl with the remaining marinade. Cover and refrigerate overnight.
To make the cashews, preheat the oven to 350 degrees F. Line a baking sheet with parchment paper. Add the cashews to a bowl. Drizzle the oil over the cashews; toss to combine. Add salt and cayenne pepper to the bowl and stir to evenly coat the cashews. Bake, stirring occasionally, until golden brown, about 15 minutes. Allow the nuts to cool completely. Store in an airtight container until ready to use.
While the chicken is marinating, make the peanut dressing by adding all of the ingredients to the bowl of a food processor or blender. Pulse until smooth. Season with salt and pepper to taste. Transfer to an airtight container and refrigerate until ready to use. The dressing will stay fresh in the fridge for 7-10 days.
To make the bahn mi veggies, add vinegar, sugar, and salt to a small sauce pan. Cook over medium heat, stirring occasionally, just until the sugar and salt dissolve. Remove from the heat and allow to cool to room temperature. While the mixture is cooling, prep the veggies and add them to a large mason jar. Pour the warm vinegar mixture over the top. Screw on the lid and give the mixture a shake to coat all of the veggies with the vinegar mixture. Place in the refrigerator for at least 30 minutes or overnight. Drain the veggies before adding them to the salad.
When you're ready to serve the salad, preheat a gas or charcoal grill. Alternatively, you can heat a cast iron pan over medium heat. Add the reserved cup of marinade to a small pot and whisk in the cornstarch. Bring the mixture to a boil over medium heat, stirring often. Allow the mixture to thicken and reduce, about 8-10 minutes. Remove from the heat and set aside. Remove the chicken from the marinade and sprinkle each side with kosher salt and pepper. Grill for 5-7 minutes on each side, brushing the chicken with the glaze as it grills, until the internal temperature reaches 160 degrees F. Allow the chicken to rest for 5-10 minutes before slicing.
To assemble the salad, add lettuce and soba noodles to a large bowl. Add red pepper, cucumbers, jalapeños, cilantro, scallions, tomatoes, drained bahn mi noodles, and roasted cashews. Season with salt and pepper and toss to combine. Add the chicken to the top of the salad. Serve with the peanut dressing.