1. Combine pineapple juice, soy sauce, ginger and salt in small bowl.
2. Place meat in resealable plastic food-storage bag. Pour pineapple mixture over meat. Refrigerate 1 hour, turning several times.
3. Heat oil in Dutch oven or pot (see Note below). Add onion; cook 3 to 5 minutes or until softened.
4. Add meat and marinade. Cover; simmer 1 to 1-1/2 hours, until tender.
5. Add celery; cook, covered, 5 minutes. Add carrots and mushrooms; cook, covered, for 10 minutes, until softened. Add spinach; cook, covered, just until wilted, 3 to 4 minutes.
6. Remove meat and vegetables to serving platter; keep warm.
7. Stir cornstarch and water in cup to make smooth paste. Stir paste into pot. Boil, stirring, until thickened, 3 minutes. Serve with meat and vegetables. Makes 6 servings.