Lingonberry & Sweet Cherry Upsidedown Cake

- 1 stick salted butter
- 1 cup packed brown sugar
- 1 cup frozen lingonberries
- 1 ½ cups frozen sweet cherries
- 1 box yellow cake mix
- 3 eggs
- 2/3 cup water
- 1/3 cup buttermilk
- 1/3 cup oil
- 1 cup chopped walnuts
- 1/3 cup granulated sugar