Preheat the oven to 350° and line 2 sheet trays with parchment paper.
In a small bowl, combine flour, oats, cinnamon, baking soda, salt and cornstarch. Set aside.
In a large bowl, cream together the butter, brown sugar, white sugar, egg and molasses. Stir until completely smooth. Add in the dry mixture and stir until evenly mixed. Add in the cranberries and walnuts and mix in.
Portion cookie dough into 2 tablespoon balls and roll in between palms to make round. It will make 15 cookies.
Space evenly on sheet trays and bake for 12-13 minutes. Press down lightly afterwards if needed. Let cool and store in an airtight container.
If using quick oats then measurement is the same. If using rolled oats, use ¼ cup more