Combine cracker crumbs and melted butter, and press onto bottom of a lightly greased 9-inch springform pan.
Bake at 350° for 8 to 10 minutes or until lightly browned. Remove to a wire rack to cool 5 minutes. Reduce oven temperature to 325°.
Beat cream cheese and 1/4 cup sour cream at medium speed with an electric mixer 1 to 2 minutes or until smooth. Add eggs, 1 at a time, beating until blended after each addition. Add onion and next 6 ingredients, beating just until blended. Fold in crabmeat. Pour mixture into prepared crust.
Bake at 325° for 40 minutes or until center is set. Let cool on a wire rack 10 minutes. Gently run a knife around edge of springform pan to loosen sides. Let cool 1 more hour.
Spread remaining 1/2 cup sour cream evenly over top of cheesecake; cover and chill at least 12 hours or up to 24 hours.
Let cheesecake stand at room temperature 30 minutes before serving. Remove sides of springform pan. Garnish, if desired. Slice cheesecake into wedges, and serve with assorted crackers.