Cut lamb into 1-inch cubes. In a large saucepan brown half of the meat in hot margarine or butter. Remove meat. Brown remaining meat with onion, celery, and garlic until vegetables are tender. Drain fat. Return all meat to pan.
Stir in tomato sauce, water, curry powder, ginger, salt, cumin, and pepper. Bring to boiling; reduce heat. Cover; simmer for 1 hour or until lamb is tender.
Combine yogurt and flour; add to meat mixture. Cook and stir until bubbly; cook and stir 1 minute more.
Meanwhile, place squash, cut side down, in a shallow baking dish. Bake, uncovered, in a 350 degree F. oven for 30 to 40 minutes or until tender. Using a fork, separate the squash pulp into strands.
To serve, spoon lamb mixture over squash. If desired, sprinkle with snipped cilantro or parsley. Garnish with cilantro or parsley sprig, if desired. Makes 4 servings.