Heat oven to 350°F. Spray 11×7-inch (2-quart) baking dish with cooking spray. In small saucepan, melt 2 tablespoons of the butter over medium heat. Stir in onion; cook 3 minutes or until tender, stirring frequently.  Set aside. In food processor, place sweet potatoes, reserved syrup and remaining 2 tablespoons butter. Cover and process 30 seconds or until smooth. Add onion mixture, beans, eggs, brown sugar, salt and 1/2 teaspoon cinnamon. Cover; process 30 seconds or until smooth. If necessary, stop machine and scrape sides. Cover and process until smooth. Spread in baking dish. To make topping, in small bowl, mix soft butter, brown sugar and flour until crumbly. Stir in pecans and cinnamon. Sprinkle on top of casserole. Bake 25 minutes or until set and temperature in center is at least 165°F. Let stand 10 minutes before serving.