1) Preheat oven to 160C/Gas 3.
2) Place the beef, water, tomato puree, sugar, vinegar, chipotle pepper, cumin, salt and black pepper in a stockpot. Cover and bake in for 1 3/4-2 1/4 hours or until beef is fork-tender.
3) Meanwhile, cut squash lengthwise into quarters and remove seeds. Place squash cut side down in a 33-by-23cm glass baking dish, overlapping if necessary. Add water. Bake in the preheated oven for 1-1 1/4 hrs or until fork-tender.
Place each baked squash quarter onto serving plate. Fill with equal amounts of beef mixture. Top with tomato and avocado, if desired. Sprinkle with coriander.
*Cook's Tip 1: 1 tablespoon minced, seeded chipotle pepper in adobo sauce may be substituted for ground chipotle chilli pepper.
*Cook's Tip 2: The chipotle beef mixture may also be cooked on the stovetop. Bring beef mixture to a boil. Reduce heat and simmer, covered, for 1 3/4-2 1/4 hrs or until beef is fork-tender.