At least 3-4 hours before you start cooking, prepare your pickled onions. Combine 1 cup of lime juice, 1/2 cup of vinegar, 1 tablespoon of sugar and 2 teaspoons of salt in a small saucepan, bring to a boil and cook until the sugar and salt dissolves. Remove from the heat and let cool for 5 minutes. Pour the liquid over the red onions in a jar or container with a lid and refrigerate.
Prepare your Kamado Joe to 325°F degrees with direct heat.
Combine 1/2 cup of honey, 1/2 cup of tamarind paste, 1/4 cup of lime juice, 8 garlic cloves, and 6 chiles in a cast iron skillet and simmer for 15 minutes.
Remove from heat and cool for 10 minutes.
After cooling, add the mixture to a food processor, season with salt and pepper, and blend until smooth. Save until later in a bowl.
Prepare your pineapple relish by brushing your pineapple slices with oil and sprinkling with salt and pepper. Grill the pineapple until charred on both sides and just cooked through. Remove from grill, dice, and place in medium bowl. Add the 4 scallions, 2 minced Serrano chiles, 1/4 cup of lime juice, and 2 tablespoons olive oil, and toss to combine. Add 1/4 cup of chopped cilantro, salt, and pepper, and toss lightly to combine. Set aside for serving.
Score the duck breasts with a sharp knife, making a diagonal cross-hatch pattern, and sprinkle with salt and pepper. Place the duck on the grill, skin-side down, and cook until the skin is nicely browned, about 15 minutes.
Once almost all of the fat has rendered and the skin is a deep golden brown, flip the duck over and brush with the tamarind glaze. When the duck reaches a 140°F internal temperature, it’s ready to remove.
Brush with glaze again and let it rest for 5 minutes before slicing on the diagonal into 1/4-inch-thick slices.
Spread a small amount of glaze on the corn tortillas. Top with sliced duck, pineapple relish, pickled red onions, and cilantro leaves for garnish.