Preheat oven to 350°
Add ½ cup of pecans to sheet pan and place in oven for 10 minutes.
Preheat 10-inch cast iron skillet for 10 minutes. Add 1 stick of butter to skillet and leave in oven until melted. Remove from oven and sprinkle 1 cup of dark brown sugar over melted butter, patting down brown sugar until it is all moistened by the butter. Do not stir. Return to oven to melt sugar, about 5 minutes.
Drain pineapple slices and reserve juice. Cut pineapple slices in half (or use whole rings, if preferred).
Finely chop toasted pecans.
Prepare cake per package directions, substituting reserved pineapple juice for water. Adding additional water if necessary, to equal amount directed.
Remove skillet from oven and arrange pineapples over brown sugar and butter in desired pattern. Add cherries in the center of each pineapple slice. Sprinkle chopped pecans in between and over the top of the arranged pineapples.
Add prepared cake mix to the skillet smooth batter with offset spatula. Bake for 30-40 minutes, until a toothpick inserted comes out with just a few moist crumbs.
Remove from oven and let rest for 10 minutes. Run knife around the edge of the skillet. Place serving platter over skillet. Using oven mitts carefully invert and remove skillet.