Easy Mixed Berry Cobbler

Bahan-bahan (8 porsi)

• 2 pounds mixed berries (fresh or frozen)
• zest from 1 lemon (optional)
• 1 tablespoon fresh lemon juice
• 3/4 cup granulated sugar
• 1/2 cup all-purpose flour
• 1/2 tablespoon cornstarch (optional)
• 1 box yellow cake mix
• 1/2 cup butter (melted (1 stick))