Heat oven to 325. 9x13" baking dish, melt butter in the oven.
Remove the baking dish from the oven after the butter is melted. Add 1 cup brown sugar and stir.
Drain pineapple in a sieve over a 4-cup measuring cup; reserving pineapple juice.
Add pineapple to brown sugar; mix and spread evenly. Sprinkle walnuts liberally over pineapple.
Add water to pineapple juice to make 1 cup; then add oil and eggs. Whisk together well.
Pour pineapple into the cake mix.
Mix well.
Pour the cake mix slowly and evenly over the pineapple mixture.
Bake until cake is done, about 40-45 minutes.
Cool slightly and turn out onto platter (scooping up any topping left in dish and spread back on the cake).
