Melt the butter in a medium saucepan over medium heat. Add the oats and toast, while stirring, until they smell nice and nutty, about 3 minutes. Add 3 cups water, the milk, brown sugar and salt. Turn up the heat, bring to a simmer and cover. Cook for 20 minutes, stirring occasionally to keep the oats from sticking to the bottom of the pan. Remove the lid and stir in the banana. Cover again and cook for 10 more minutes, stirring on occasion, until the oats are soft and creamy. Stir in the berries right before serving.