Preheat oven to 375 degrees.
Measure oats into a very large bowl. Pour milk over top to let soak in.
Add mashed banana, cinnamon, vanilla, sugar, flaxseed and brewer's yeast.
Stir until combined and wet. Gently stir in white chocolate chips.
Divide oatmeal between two greased 8x8 baking pans. Press oatmeal down in each pan to make a flat, even top.
Bake 16-18 minutes (the longer you bake, the more dry your oatmeal will be, so it's your preference).
Let cool, then cut each pan into 9 squares.
You can freeze 1/2 of the oatmeal after making. Just make sure it is sealed well before you put it in the freezer, or it may dry out.