Cut each zucchini in half crosswise. Using a melon baller, carefully scoop out zucchini flesh, leaving about 1/4 inch flesh intact on all sides and leaving zucchini closed at the bottom of each half. Reserve zucchini flesh for another use, or discard. Reserve zucchini tubes.
Heat oil in a heavy skillet over medium-high heat. Add onions and cook, stirring, until they are soft and translucent, about 3 minutes. Add garlic and cook for 2 minutes more. Add ground beef, cumin, coriander and red pepper flakes and cook until meat is just done and no longer pink, 4 to 5 minutes. Add rice, and diced tomatoes and cook an additional 2 to 3 minutes. Remove from heat and stir in pine nuts and parsley. Let mixture cool slightly, about 5 minutes. Using a small spoon or clean hands, gently stuff zucchini halves with beef-mixture.
Place zucchini halves in a 9 by 13-inch glass baking dish. Combine reserved tomato juice, tomato sauce and salt and pour over zucchini. Cover tightly with foil and place in oven. Bake for 20 minutes. Remove from oven, uncover and rotate zucchini. Re-cover, return dish to oven and bake an additional 20 to 25 minutes, until zucchini is cooked through but still slightly firm. Place 2 zucchini halves on a plate and serve with tomato sauce.
1 serving is 2 zucchini halves and 3 tablespoons tomato sauce