1. Make the mojo de ajo: Cut the stems from the chilies and remove the seeds. Toast the chilies, peppercorns, and oregano in a skillet set over medium heat, until just fragrant. Transfer to a spice grinder or small food processor and process to a powder.
2. Pour olive oil in a saucepan and add the garlic. Set over medium heat and cook, stirring, until the garlic softens and turn light brown.
3. Add the tomato and orange juice, along with the processed chile-herb blend, and cook another two minutes. Let cool before using.
4. Prepare the grill by lighting the coals, or preheating to high. Once hot, prep the grates by wiping them with a paper towel soaked in vegetable oil.
5. Make three to four diagonal slices through the duck skin. Cut the onion in thick slices. Brush both the duck and the onion with oil from the mojo de ajo. Season with kosher salt and let rest while the grill heats.
6. Once the coals are ready or the grill is hot, place the duck, skin side down, and onions on the grill and cook for 2-3 minutes. Flip and cook another 2-3 minutes, or until the duck is medium rare.
7. Transfer the duck and onions to a cutting board and let rest for 5 minutes. Chop the duck and onion together. Strain two tablespoons of garlic from the mojo de ajo and add to the duck-and-onion mixture.
8. Serve tucked into warm tortillas, garnishing with a squeeze of lime juice, queso fresco, and pickled red onions.