Preheat the oven to 350°F (175°C). Lightly grease a well-seasoned 10-inch cast iron skillet.

Combine dry ingredients: In a medium mixing bowl, whisk together the flour, rolled oats, baking soda, ground cinnamon, and salt. Set aside.

Cream butter and sugars: In the bowl of a stand mixer fitted with the paddle attachment, or using a hand mixer, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy, approximately 3 to 5 minutes.

Incorporate eggs and vanilla: Add the eggs one at a time, mixing well after each addition. Add the vanilla extract and continue to mix until fully incorporated.

Combine wet and dry ingredients: Gradually add the dry mixture to the wet ingredients, mixing until just combined. Avoid overmixing.

Fold in chocolate and nuts: Using a spatula, gently fold in the semisweet chocolate chips and chopped pecans until evenly distributed throughout the dough.

Transfer to skillet: Spoon the cookie dough into the prepared skillet and use your hands or a spatula to press it into an even layer.

Bake: Place the skillet in the preheated oven and bake for 35 to 40 minutes, or until the top is firm to the touch and the edges have begun to pull away from the sides.

Cool and serve: Allow the cookie to cool in the skillet for approximately 10 to 15 minutes. Run a spatula gently around the edges to loosen it. Carefully invert onto a cutting board, then again onto a serving platter. Slice and serve warm, optionally topped with ice cream or sauce.