In a large mixing bowl, beat the egg, butter, brown sugar, sugar, vanilla, salt, and baking soda. Add the flour and oatmeal, beating just until mixed.
Stir in the trail mix and dark chocolate chips, stirring until the trail mix is completely coated in the dough.
Cover the bowl and refrigerate for 2 hours.
Preheat the oven to 350 degrees. Prepare a baking sheet with parchment paper.
Using a large cookie scoop or ice cream scoop, portion balls of cookie dough and place on baking sheet about 2 inches apart.
Using a damp hand, press the cookies to about 1 inch thickness. (Be aware that the cookies will spread slightly when baking.)
Bake the cookies for 12 to 15 minutes, or until your desired level of crunch is achieved. (For chewy cookies, cook less. For crunchier cookies, cook longer.) Allow the cookies to cool completely on the baking sheet before removing from parchment paper.