Preheat oven to 375º
Combine flour, rolled oats, baking powder and salt. Cut in butter with pastry blender until mixture resembles coarse crumbs. Stir in rhubarb by hand to coat pieces.
In a large bowl, stir together mashed bananas, brown sugar, sourdough starter, egg, almond extract and lemon zest. Add the dry to the wet and stir just until moistened. Turn dough out onto a well floured surface and pat into a 12 inch round. Dough will be moist and sticky, so make sure the surface is well floured. Divide dough into 8 to 10 scones. Transfer to a baking sheet lined with parchment paper. Bake at 375º for 25 minutes.
While scones are baking, make glaze.
Combine confectioners' sugar, lemon juice, and milk until desired consistency.
Remove scones from oven and transfer to a wire rack to cool slightly.
Drizzle scones with lemon glaze while warm, top with sliced almonds, optional