Preheat oven to 350 degrees. Melt 3/4 cup butter in a heavy 10 inch skillet. Sprinkle 3/4 cup brown sugar and 1 (3 ounce) package strawberry gelatin over butter. Drain 1 (8 1/4 ounce) can crushed pineapple, reserving liquid. Spread crushed pineapple and 1/2 cup chopped walnuts over gelatin mixture. Prepare 1 yellow cake mix according to directions on package. Using reserved pineapple juice plus water to measure 1 1/3 cups liquid. Pour cake batter over pineapple mixture. Bake for 50-55 minutes. Immediately turn upside down cake onto a serving dish.