1/4 cup soy sauce
1/4 cup fish sauce
3 tablespoons rice vinegar
3 tablespoons firmly packed dark brown sugar
2 tablespoons peeled and minced fresh ginger
1 tablespoon finely minced garlic
2 pounds boneless leg of American lamb, cut into 1-inch cubes
1 English cucumber
1 large shallot, halved lengthwise and thinly sliced crosswise
2 to 4 red Thai chiles or jalapeños, thinly sliced
1/4 cup plus 2 tablespoons rice vinegar
1/4 cup sugar
1/2 teaspoon salt
In a medium bowl, whisk together the soy sauce, fish sauce, vinegar, sugar, ginger, and garlic. Thread the lamb pieces onto wooden skewers, leaving about one-third of each skewer free to use as a handle. (Don’t leave gaps between the meat pieces, but don’t smush them together too tightly, either, or the lamb will cook unevenly.) Arrange the assembled skewers in a shallow baking dish in a single layer and pour the marinade over, turning the skewers to coat the lamb evenly. Cover and let marinate in the refrigerator for at least 4 hours and up to 24 hours, turning once about halfway through.
To make the relish: Halve the cucumber and then halve each half again lengthwise. Quarter each piece lengthwise, so you end up with long wedges (like skinny pickle spears). Thinly slice the wedges crosswise into small triangles. Combine the cucumbers with the shallots, chiles, vinegar, sugar, and salt in a small bowl. Cover and refrigerate for at least 1 hour and up to 1 day before serving. (Note, the cucumbers will soften a bit if left to marinate for more than a few hours, but are still delicious.)
Prepare a hot fire in a charcoal grill or preheat a gas grill to high. Lightly oil the grill grate. Remove the lamb skewers from the marinade, gently shaking off the excess, and arrange them on the hottest part of the grill. Grill, turning once, until nicely charred on the outside but still slightly pink in the center, 3 to 4 minutes per side for medium. Transfer to a serving platter and garnish with peanuts and cilantro leaves. Serve hot or at room temperature with the cucumber relish on the side.