Carnitas Tacos with Pickled Red Onion

The taste of pork is greatly enhanced by achiote paste, a mixture of ground annatto seeds, vinegar, salt and spices. Find it in Latin markets.
Preheat the oven to 250 degrees.Heat a large cast-iron skillet over medium-high heat. Add the garlic to the skillet; cook for 8 minutes or until the skin is lightly browned, turning occasionally. Remove the cloves from the pan; cool for 5 minutes and peel. Combine the garlic, zest, juice, achiote paste, and 2 teaspoons of oil in a mini chopper; grind until smooth.Sprinkle the pork with 1 teaspoon of salt. Brush the pork evenly with the achiote mixture. Place the pork in a medium-sized roaster or heatproof pan; cover and bake at 250 ° C for 6 hours. Remove from the oven and let stand for 30 minutes.While the pork is cooking, place the onions in a medium bowl. Combine 1/2 cup water, vinegar, agave, and remaining 3/4 teaspoon salt in a saucepan over medium-high heat; bring to a boil, stirring, until the salt is dissolved. Drizzle the onion mixture with vinegar; toss to combine. Cover the bowl with plastic wrap; cool to room temperature, rearranging as necessary to allow the onions to remain submerged. Refrigerate for at least 1 hour.Place the pork on a cutting board; chop with 2 forks. Cut out the fat.Heat a large cast-iron skillet over medium-high heat. Add 2 tablespoons of oil to the skillet; stir to coat. Place 1 cup minced pork in an even layer in the pan; cook for 3 minutes or until crisp, turning once. Place the toasted pork on a platter. Repeat the procedure with the remaining shredded pork.Place 2 warm tortillas on each of 6 plates. Top each tortilla with 1 1/2 ounces toasted pork, 1 tablespoon tomatillo salsa, and 1 tablespoon pickled red onion.