Step 1Combine the lemongrass, shallot, four-fifths of the sliced garlic, half the sliced chiles, ¼ cup fish sauce and ¼ cup palm sugar in a large, shallow dish and stir until the sugar dissolves. Add the pork chops and turn to evenly coat, spooning the solids on top of the meat. Or you can pour the mixture into a resealable plastic bag, add the pork chops and seal the bag. Let stand at room temperature while the grill heats, at least 10 minutes.
Step 2Set up a charcoal grill for direct, high-heat grilling or heat the burners of a gas grill to medium-high. (Alternatively, heat a large skillet or grill pan over medium-high heat on a stove.)
Step 3While the grill heats, stir the lime juice, hot water and the remaining sliced garlic, sliced chile, ⅓ cup fish sauce and 3 tablespoons sugar in a medium bowl until the sugar dissolves. Stir in the watermelon radish and let stand at room temperature until ready to serve.
Step 4Remove the pork from the marinade, brushing off the solids, and place the chops on the hot grill. Cook until the bottom of the meat has charred and releases easily from the grate, about 5 minutes. Flip the chops and grill until the other side is charred and the meat has lost almost all of its pink color in the center, 3 to 5 minutes.
Step 5Transfer the pork chops to serving plates, spoon some of the radish nuoc cham on top, and let rest for 5 minutes. Serve with the remaining nuoc cham, and serve alongside the tomatoes, cucumbers, and herbs.