Over low heat, melt butter in a 12" oven safe skillet. Stir in brown sugar until blended. Remove from heat.
Drain pineapple over a bowl, saving the juice for later on. Arrange the pineapple in an even layer over the butter and brown sugar mixture. Sprinkle maraschino cherries over top of the pineapple and chopped nuts over top of the cherries.
Pour the pineapple juice into a measuring cup and add water to the juice to measure 1 1/3 cups.
In a large bowl, whisk together cake mix, pineapple juice, eggs, and oil until smooth and well blended. Pour over the layers in the skillet.
Place entire skillet in the oven and bake for 35-40 minutes or until a cake tester comes out clean.
Allow to cool on a cooling rack or the stove top for 5-10 minutes. Hold a large platter or plate over the skillet. Holding the platter tightly against the skillet, turn the cake over. Slight jiggling may be required, but the cake will fall out onto the platter.
This cake is baked in a large skillet. You'll need one that is oven-safe up to at least 350. I like to use my 12" stainless steel skillet, but you could also use a 12" cast iron skillet.Pineapple rings look the nicest, but you can use slices, chunks, or crushed if you prefer.