Since this is baking I will give you more precise ingredient amounts.
Heat the oven to 375.
Coat 8 x 8 inch pan with non-stick cooking spray.
Mix 1 can (14oz) sweetened condensed milk and 3 tbls grated lemon zest and 1/4 cup fresh lemon juice in a medium bowl and set aside.
In another medium bowl mix the following until crumbly:
1 1/4 cup flour, 1 cup old fashioned oats, 1/2 cup pack brown sugar, 1/2 cup softened unsalted butter, 1/4 teasp baking soda and 1/4 teasp salt.
Press half of the crumbly mixture in pan, bake about 10 minutes or until set.
Spread the lemon milk mixture over the baked crust and sprinkle the remaining crumbly mixture over the lemon milk mixture and press down gently.
Bake about 20 minutes or until the edges are golden brown and the center is set but soft. Cool completely and cut in 4 rows by 4 rows. Perfect!