Preheat the oven to 350°F / 176°C.
Line two baking sheets with parchment paper and set them aside.
In a medium mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, ground cinnamon, ground ginger, allspice, ground cloves, ground nutmeg, and salt.
Set the dry mixture aside.
In a separate mixing bowl, vigorously whisk the melted butter and packed brown sugar until smooth and fully combined.
Add the room-temperature egg and vanilla extract to the butter mixture.
Whisk until the wet ingredients are fully incorporated.
Sift the dry ingredients into the wet ingredients.
Stir with a spatula until just combined and no visible flour streaks remain.
Fold in the old-fashioned oats and chopped pecans until evenly distributed throughout the dough.
Cover the mixing bowl and refrigerate the cookie dough for 15 minutes.
Using a 1.5 or 2-tbsp cookie scoop, portion the dough onto the prepared baking sheets.
Space the dough balls evenly apart to allow room for spreading.
Bake for 12 to 15 minutes, or until the bottoms are lightly golden and the centers appear slightly under-baked.
Remove the baking sheets from the oven.
Allow the cookies to cool on the baking sheets for several minutes before transferring them to a wire rack.
Let the cookies cool completely before serving.