Bring water and milk just to a boil in a medium saucepan. Reduce heat to medium low and stir in oats, buttermilk, mashed banana, brown sugar, cinnamon and salt. Cook 5 more minutes, stirring occasionally. Remove oatmeal from heat, stir in vanilla and optional butter or cream. Cover with a lid and let rest 3 minutes. Serve warm topped with optional chopped pecans.
For alternate cooking method: In a very large microwave safe bowl, combine oats, milk, buttermilk, water, mashed banana, brown sugar, cinnamon and salt. Microwave on high for 3 1/2 minutes. Allow to rest in the microwave for 2 minutes. Remove from microwave and stir in vanilla, optional cream or butter and optional pecans. Serve warm.
Notes
*Depending on how ripe your bananas and how sweet you like your oatmeal.