Sourdough starter needs to be prepared several days ahead.
Cherry FIlling
Bring 1-1/2 cups fresh or frozen unsweetened pitted tart red cherries to boiling; reduce heat. Cover and simmer for 5 minutes. Stir in 1 tablespoon lemon juice. Combine 1/2 cup sugar and 2 tablespoons cornstarch; add to cherry mixture. Cook and stir till bubbly. Cook and stir 2 minutes more. Cool completely.
Preheat oven to 350°.
Mix the 1-1/2 cups flour, the sugar, baking powder, baking soda, and 1/4 teaspoon salt. Cut in 1/2 cup margarine till mixture resembles fine crumbs.
Prepare Sour Dough Starter.
Mix Sourdough Starter, egg, and vanilla; add to flour mixture. Stir just till moistened.
Spread half of the batter in a greased 9 x 9 x 2-inch baking pan. Spread Cherry Filling atop. Drop remaining batter in small mounds over filling.
Mix oats, brown sugar, nuts, and the 3 tablespoons flour. Cut in 1/4 cup margarine till mixture resembles coarse crumbs; sprinkle over batter.
Bake in a 350° oven for 35 to 40 minutes or till golden.