1. Preheat the oven to 400 degrees.
2. In a small roasting pan, combine the cubed pork tenderloin, enchilada sauce, the juice of the orange, and the remaining (juiceless) orange as well. Season with salt and pepper, toss together, cover with foil and place in the oven for 2 hours.
3. Now, slice off the top of the head of garlic to expose the cloves. Place on a piece of tin foil, drizzle with olive oil, and season with salt and pepper. Tightly wrap closed and let cook for 30-60 minutes in the oven, until the cloves have caramelized and are starting to burst out of the head; remove from the oven and let cool.
4. Next, prepare the sauce: Halve the tomatoes lengthwise. In a dry sauté pan, place the tomatoes cut side down and cook on high until the tomatoes are charred. When tomatoes are blackened, transfer to a blender. To the same hot pan, then, add the guajillo chiles and toast until you can smell them. Add the chicken broth to the pan and simmer with chiles for 2 minutes. When the chiles are tender from the chicken broth, pour them both into the blender. Squeeze in the juice of the lime, all the cloves of the roasted bulb of garlic, season with salt and pepper, and blend to a fine consistency. Set aside until ready to use.
5. After 2 hours, check on the pork tenderloin. If it is tender enough to shred with a fork, remove from the oven, and let cool. While it cools, cube your avocado and add to the pork. Remove the rind and supreme the orange and add to the pork. Toss everything together while gently shredding the pork with a fork.
6. To assemble the burritos, spread 1/4-cup of pork filling into each tortilla. Fold the sides inward and the tightly roll the burritos closed. Repeat until you have used all of the pork.
7. In a dry sauté pan, place the burritos seam side down and toast for 1-2 minutes, until golden brown. Flip and toast on the second side. Before you serve, generously spoon over the charred chili tomato sauce and drizzle with a sour cream with lime zest.