Mash the bananas. Mash the bananas well, so there are no large lumps. Add the brown sugar and nutmeg and stir them all together. Add the buttermilk and eggs. Using a rubber spatula, combine everything together until the eggs are fully incorporated.
Combine the dry ingredients. Whisk together the flour, baking powder, baking soda and salt in a large bowl. Make a well in the center of the flour.
Make the pancake batter. Pour the wet ingredients into the well and mix together until there are no visible signs of flour. Add the walnut pieces and stir again. Let the batter sit for 30 minutes before making the pancakes.
Make the pancakes. Heat a nonstick pan or electric griddle over medium heat. When the pan is hot, add some butter or oil. Drop 1/4 cup pancake batter into the pan and repeat to make 3 more pancakes. Cook until bubbles have formed and started to pop on the top of the pancakes, about 2 minutes. Flip the pancakes and cook until the bottom is cooked, about 1 minute.
Put a stack of pancakes on a plate and cook the remaining pancakes. (If you want to keep the pancakes warm, put the plate in a 200 degree F oven as you cook the remaining pancakes.) Top with banana slices and maple syrup.