Stir flour with baking soda and salt in a small bowl. In another, larger, bowl whisk egg, brown sugar, buttermilk, oil and vanilla together until blended. stir in the flour mix.
Add rhubarb and pecans (if using) and stir just until combined. Scrape into the prepared pan.
Bake in center of oven for 70 - 75 mins or until a tester inserted into cake center comes out clean. Cool in pan for 10 minutes Remove from pan.
While cake is cooling in the pan stir icing sugar with lemon juice in a small bowl, adding the juice 1 tsp at a time until thick and smooth. Drizzle the glaze over the warm cake when it's removed from the pan letting the glaze run down the sides.