Preheat the oven to 375 degrees F. In a small bowl, whisk together the milk, juice and lemon zest. Set aside.
In another bowl, sift together the flour, baking powder and salt. In a 3rd bowl, use an electric mixer to cream the butter and blend in the sugar. Add the oats and flour mixture and mix until crumbly. Spread half of the mixture in the bottom of a 9x12-inch greased baking pan. Pat the mixture firmly into the bottom of the pan. Spread the liquid mixture over the top and cover with the remaining crumb mixture.
Bake for 20 to 25 minutes or until the edges are browned. Cool in the pan at room temperature for 15 minutes.
Cut into 2-inch squares and chill in pan until firm. Remove squares from the pan when firm.
Be careful! If you try to remove the squares from the pan too early, they will fall apart.