Using a sharp knife, cut 24 meatballs into thin slices. Divide the meatball slices evenly among the 4 bottom halves of bread. Top each sandwich bottom with 1 sliced green onion, sliced Vietnamese Pickled Carrots and sliced peppers, to taste. Sprinkle with Nuoc Cham Sauce and then spoon more Nuoc Cham Sauce, to taste, over the top cut portions of the bread. Spread bread with mayonnaise, if desired. Place the sandwich tops over the meatballs and serve the sandwiches, cut in half if desired. Place any remaining Nuoc Cham in a small bowl and pass at the table, if desired. Garnish with fresh sprigs of cilantro and mint, if desired.
In a small bowl combine the pork, shallots, garlic, sugar, fish sauce and pepper. Stir to combine well, cover, and refrigerate for at least 1 hour and up to overnight. Transfer to the freezer for 30 minutes, or until the mixture is partially frozen.
Place the rice in a small skillet and heat over medium-high heat, stirring constantly, until rice is toasted, golden brown and fragrant. Transfer to a plate to cool. When cool, place rice in a coffee grinder and process to a fine powder. Measure 3 tablespoons of the powder and set aside. Save any remaining powder for another purpose or discard.
Transfer the partially frozen meat mixture to a food processor and process to a completely smooth but stiff paste. Add the pork fat to the processor and process until smooth or finely chopped. Add the roasted rice powder and pulse several times to combine the mixture. Do not overprocess or the mixture will become sticky.
Preheat a grill to medium-low.
Transfer the meat mixture to a small bowl. Lightly oil your hands. Divide meat mixture into heaping 1 1/2 tablespoonfuls and roll each into a smooth ball. Recoat your hands with oil as necessary. Thread the meatballs onto the bamboo skewers, fitting as many as you can on each skewer.
Coat the grill lightly with oil and grill the skewered meatballs, turning occasionally, until cooked through, 20 to 30 minutes. Serve warm or at room temperature.
Combine the garlic, chile and sugar in a mortar and pound with a pestle to a fine paste. Add the lime juice, vinegar, fish sauce and water and stir to blend. Stir in the carrot and serve immediately or refrigerate, covered and in a non-reactive container, for up to 2 days.
Yield: about 1 1/4 cups
Place the carrots in a small nonreactive bowl.
In a small saucepan combine vinegar, sugar and salt and bring to a boil. Remove and cool slightly. Pour warm vinegar mixture over carrots and allow to marinate for at least 1 hour and up to 2 weeks, refrigerated.