Preheat oven to 325 degrees. Season brisket generously with salt and pepper.
Preheat a large dutch oven over medium heat. Place brisket in pot, fat side down, and sear on all sides until deeply golden brown, about 4-5 minutes per side. Remove brisket to a plate.
Add 2 tablespoons olive oil, carrot, onion, celery, pepper, and garlic. Season with salt and pepper and and saute? until softened and beginning to caramelize, about 6 minutes. Add coriander, cumin, paprika, cayenne, and oregano and toast, stirring, until fragrant, about 1 minute. Add tomato paste and cook, 1-2 minutes, before adding tomatoes and beef stock. Bring to a brisk simmer, return meat to pot, and cover. Transfer to oven and braise, flipping brisket every 45 minutes, until meat is tender and shreds easily with two forks, about 4-5 hours. Remove meat from sauce and keep lightly covered.
Return dutch oven to stove top over high heat and cook, stirring frequently to prevent scorching, until sauce is reduced by half, about 10 minutes. Reduce heat to low and return meat to sauce. Using two forks, pull meat into long shreds. Fold to coat with sauce and keep warm until ready to serve.
For Pesto Guac:
Combine avocados, lemon juice, and olive oil in a medium bowl. Mash with a fork or potato masher until desired consistency is reached. Add basil and parmigiano-reggiano. Fold to combine, and season to taste with salt and pepper. Keep covered in the refrigerator until ready to serve.
For Apple-Pear Slaw:
Combine apple, pear, cabbage, apple cider vinegar, and olive oil in a large bowl. Toss to combine and season to taste with salt and pepper. Keep covered in the refrigerator until ready to serve.
To serve, warm tortillas, in batches if necessary, in a cast iron skillet on medium heat for 30 seconds to 1 minute per side. Transfer to a dish towel to keep warm. Serve with brisket and sauce, guac, slaw, cilantro, lime wedges, yogurt, and hot sauce.