Prepare Pulled Pork: In large bowl combine ketchup, onion, cider vinegar, molasses, cumin, garlic powder, oregano and Tabasco. Add pork pieces: toss to mix well.
Place mixture in slow-cooker. Cook on high 5 hours or cook on low 8 hours, stirring occasionally until pork is tender.
Prepare Sandwiches: With two forks, shred pork; return to cooking liquid; toss to mix well. Cut each roll in half; fill with pulled pork mixture. Top with tomato and avocado slices and crumbled queso fresco.