Preheat the oven to 375°F (190°C). If you have a fan or convection oven, the temperature will need to be lower I set mine to 175°C (350°F).
Cream the butter and sugar together in a large bowl until thoroughly mixed. You can use a wooden spoon or an electric mixer.
Beat in the egg and vanilla.
Add the mashed banana.
In a separate large bowl, mix the flour, baking soda, salt, cinnamon and nutmeg together. I like to do this by sifting, but flour doesn't generally need to be sifted these days, so you can just mix it together if you prefer.
Stir the oatmeal and raisins into the flour mixture.
Add these dry ingredients to the wet ingredients in the first large bowl.
Drop rounded teaspoons of dough on an ungreased baking sheet, about 1½ inches (3.5 cm) apart.
Bake for 12 to 15 minutes.
Cool on a wire rack, although these cookies do taste amazing warm so feel free to eat them as soon as they are safely cool enough to eat.
When the cookies are completely cool, store in an airtight container. They taste best served within a day or so of baking.