Preheat oven to 350°.
Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, sugar, and salt in a bowl; cut in margarine with a pastry blender or 2 knives until mixture resembles coarse meal. Reserve 1/2 cup flour mixture for topping; set aside.
Combine remaining flour mixture, rosemary, baking powder, and baking soda; add buttermilk, lemon juice, and egg. Beat at medium speed of a mixer until blended. Spoon batter into an 8-inch round cake pan coated with cooking spray. Combine reserved 1/2 cup flour mixture, lemon rind, and water; stir with a fork. Sprinkle crumb mixture over batter. Bake at 350° for 30 minutes. Cool on a wire rack. Garnish with rosemary sprigs and lemon slices, if desired.