Heat oil in a nonstick skillet over medium-high heat; add pork loin. Brown pork on all sides (about 11 minutes); deglaze pan with stock scraping any brown bits. Place pork mixture and cooking liquid into slow cooker. Cover and cook on LOW 7 to 8 hours or HIGH 3 to 4 hours. Remove pork from cooking liquid. Pour cooking liquid into a large skillet and bring to a boil over high heat; boil 5 minutes or until liquid is reduced to about 1/3 cup. Shred pork; stir in reduced cooking liquid. Serve with Chimichurri Sauce, warm tortillas, lettuce and sour cream.
To make Chimichurri Sauce, combine all ingredients (parsley, oregano, garlic, shallots, lime juice, mayonnaise, red pepper flakes) and 1/4 cup water in a food processor or blender. Pulse 8 to 10 times or until finely chopped, stopping to scrape down sides as needed. Makes about 1 1/4 cups.