Heat the oven to 400°F.

• Cut the cucumber in half lengthwise; cut one half on the diagonal into ¼-inch-thick slices. Save the remaining half for another use. [Cut 1 cucumber in half lengthwise, then on the diagonal into ¼-inch-thick slices. Save the remaining cucumber for another use.]

In a small sauce pot, bring the quick-pickle brine and ¼ cup [½ cup] water to a boil. Remove from the heat, add the cucumber and carrots, and season with salt. Cover and let stand, stirring occasionally, while you prepare the rest of the meal. 

• Ground meat (including plant based): Cut a small corner from the ground meat packaging and drain off any excess liquid. Transfer to a plate; pat dry with a paper towel.

In a large bowl, combine the meat and lemongrass paste, season generously with salt and pepper, and mix until just combined. Using wet hands, form the mixture into 2-inch meatballs. 

In a large frying pan over medium-high heat, warm 1 tablespoon [2 TBL] oil until hot but not smoking. Add the meatballs; using the back of a spatula, press them lightly into patties about ¾ inch thick. Cook, turning once, until browned and cooked through, 3 to 4 minutes per side. Transfer to a plate. Do not clean the pan. While the meatballs are cooking, prepare the ciabatta.

• Cut the ciabatta in half horizontally; drizzle or brush the cut sides with 1 teaspoon [2 tsp] oil and season with salt.

On a sheet pan, place the ciabatta, cut sides up, and toast in the oven, turning once, until lightly browned and crisp, 8 to 10 minutes. Meanwhile, prepare the sauce and finish the meatballs. 

In the same pan used for the meatballs over medium heat, add ⅓ cup [⅔ cup] water, scraping up any browned bits from the bottom of the pan. Stir in the cashew butter and cook until incorporated and the sauce is thickened slightly, about 1 minute. Add the meatballs and stir to coat. Remove from the heat. 

• Remove any coarse stems from the cilantro sprigs. 
• If using the jalapeño, remove the stem, ribs, and seeds; thinly slice the jalapeño. Wash your hands after handling.

Using a slotted spoon, remove the pickled vegetables from the brine, reserving the brine. Place the ciabatta bottoms, cut sides up, on a work surface. Top with the meatballs and sauce, pickled vegetables, cilantro sprigs, and as many jalapeño slices as you like. Drizzle with the brine. Close with the ciabatta tops.

Transfer the banh mi to individual plates and serve any remaining pickled vegetables and jalapeño slices on the side.