In a large bowl, whisk together flour, sugar and baking powder. Add egg, milk, butter and lemon peel; mix just until dry ingredients are moistened. Fold in the blueberries. Spread in a greased 9-in. square baking pan.
For topping, combine sugar, flour, walnuts and cinnamon in a mini food processor or bowl. Add butter and process, or cut in if doing by hand, until mixture is crumbly. Sprinkle over batter. Bake at 350 degrees F for 40-45 minutes or until cake tests done.
For drizzle, combine the powdered sugar and milk. If too thick, add a few drops of additional milk at a time until it reaches drizzling consistency. Drizzles over the top of the cake and allow to sit until sugar solidifies.
Notes
You can substitute hazelnuts for walnuts.
For the drizzle, more or less milk may be required for drizzling consistency.
This cake has a very thick batter that will have to be spread into the pan rather than poured. It's also very important to make sure you are measuring your flour properly so that you don't end up with more than you are supposed to. Always measure flour using the "scoop and sweep" method.