Preheat oven to 325 degrees. Place beef, water, tomato paste, sugar, vinegar, chipotle pepper, cumin, salt and black pepper in stockpot. Cover and bake for 1-3/4 to 2-1/4 hours or until beef is fork-tender.
Meanwhile, cut squash lengthwise into quarters; remove seeds. Place squash cut side down in 13-by-9-inch glass baking dish, overlapping if necessary. Add water. Bake for 1 to 1-1/4 hours 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 cilantro.
Cook’s tips: One tablespoon minced, seeded chipotle pepper in adobo sauce may be substituted for ground chipotle chile pepper. Chipotle Beef mixture may also be cooked on the stovetop. Bring beef mixture to a boil. Reduce heat; simmer, covered, 1-3/4 to 2-1/4 hours or until beef is fork-tender.