Preheat the oven to 350°F. Butter the bottom and sides of a 9-by-13-inch baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up the two short sides of the pan and overhang slightly on both ends. (This will make it easy to remove the bars from the pan after they have baked.) Butter the parchment.
In a medium bowl, combine the milk and lemon juice; set aside.
In a large bowl, whisk together the flour, oats, brown sugar, baking powder, salt lemon zest and butter. Mix until crumbly.
Press half of the oatmeal mixture into the prepared pan and use your hands or the back of a large wooden spoon to push the crust into an even layer at the bottom of the pan.
Spread the lemon filling evenly on top of the crust. Sprinkle the reserved oatmeal mixture evenly on top of the filling.
Bake for 25 to 30 minutes, until the top is golden brown. Transfer to a wire rack to cool completely, then cut into squares and serve. The bars can be stored in the refrigerator in an airtight container for up to two days.