Add the vegetable stock, tomatoes, chipotle peppers, onions and garlic to a Dutch oven and stir to combine.
In small bowl, combine the cumin, chili powder, salt and black pepper. Sprinkle the spice mixture over the beef so that it is evenly coated. Put the seasoned beef into the Dutch oven and cover. Braise in the oven until the meat is fork tender, about 3 hours. Remove the beef from the pot and put it into a baking dish. Shred with 2 forks and drizzle with some of the braising liquid to keep the meat moist. Cover with some aluminum foil until ready to serve.
Preheat a broiler.
Put the poblano peppers on a sheet pan under the broiler for 8 minutes, turning every 2 minutes until the skins are charred. Remove the pan from the broiler, put the peppers into a bowl, cover with plastic wrap and let them sit to allow the skin to separate from the flesh, about 10 minutes. Core, seed and peel the charred skin from the peppers.
Add the peppers along with the remaining salsa ingredients, except 3/4 of the diced avocado, to a blender. Puree the mixture until smooth. Pour the mixture into a medium bowl, and add the remaining diced avocado. Gently toss the salsa with a rubber spatula to coat the avocado. Taste and adjust the seasonings with more salt and pepper, if needed. Set aside
Bring the canola oil to 350 degrees F in a large skillet over medium heat. Fry the corn tortillas in batches until crispy but still slightly pliable, about 2 to 3 minutes. Transfer them to a sheet tray lined with paper towels to drain any excess oil.
Put the flour tortillas in a damp towel and microwave for 45 seconds on high to soften them. Arrange a fried corn tortilla onto each flour tortilla. Spoon some of the beef onto the fried tortilla, top each taco with some shredded cabbage, 2 tablespoons green salsa and 1 tablespoon queso fresco. Repeat with the remaining ingredients. Arrange on a serving platter and serve.