Baked Oatmeal

• 2 cups/7 oz/200 g rolled oats
• 1/2 cup/2 oz/60 g walnut pieces, toasted and chopped
• 1/3 cup/2 oz/60 g natural cane sugar or maple syrup, plus more for serving
• 1 teaspoon aluminum-free baking powder
• 1 1/2 teaspoons ground cinnamon
• Scant 1/2 teaspoon fine-grain sea salt
• 2 cups/475 ml milk
• 1 large egg
• 3 tablespoons unsalted butter, melted and cooled slightly
• 2 teaspoons pure vanilla extract
• 2 ripe bananas, cut into 1/2-inch/1 cm pieces
• 1 1/2 cups/6.5 oz/185 g huckleberries, blueberries, or mixed berries