Melt the margarine in a large nonstick skillet over medium-high heat. Add quince; sauté 3 minutes or until lightly browned. Add 1/2 cup broth and onion. Simmer 12 minutes or until liquid evaporates. Add ginger, jalapeño, and garlic; sauté 1 minute. Add curry powder; sauté 1 minute. Add 1 cup broth and tomatoes. Bring to a boil; reduce heat, and simmer 15 minutes. Add bell peppers and currants; simmer 5 minutes. Add shrimp; simmer 4 minutes.
Combine water and cornstarch. Stir yogurt until smooth. Add the cornstarch mixture, yogurt, salt, and pepper to skillet. Bring to a simmer, and cook 1 minute or until mixture is slightly thick, stirring constantly. Serve the curry over rice, and sprinkle with green onions.