Preheat oven to 325°F. Place beef, water, tomato paste, sugar, vinegar, chipotle pepper, cumin, salt and black pepper in stockpot. Cover and bake in 325°F oven 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 x 9-inch glass baking dish, overlapping if necessary. Add water. Bake in 325°F oven 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.
Nutrition information per serving: 355 Calories; 81 kcal Calories from fat; 9 g Total Fat; 3 g Saturated Fat; 3 g Monounsaturated Fat; 73 mg Cholesterol; 528 mg Sodium; 44 g Total Carbohydrate; 9.7 g Dietary Fiber; 29 g Protein; 5.3 mg Iron; 7 mg Niacin; 0.7 mg Vitamin B6; 2.2 mcg Vitamin B12; 6.3 mg Zinc; 20.5 mcg Selenium;