Toast chopped walnuts in a dry skillet or the oven until lightly browned, set aside to cool.
Heat a 10" cast iron or other heavy-bottom skillet over medium high heat.
Melt the butter, then add the cinnamon, cardamom, and nutmeg and allow the spices to bloom for about 45 seconds to a minute.
Add the brown sugar and vanilla. Cook whisking constantly until the sugar melts and the sauce becomes homogenous.
Add the bananas cut side down and allow to cook for about 1 minute until lightly browned, flip and cook another minute on the other side. When you flip the bananas sprinkle the cut side with a little kosher salt - this will not make the dish salty it will just wake up all the flavors.
Add the rum - if using a gas stove top you might want to turn off the flame and/or remove the pan from the stove.
Carefully light the rum with a long lighter or long kitchen matches and shake the pan while the alcohol burns off.
Once the flame subsides let the banana foster cool slightly and serve.
To serve either spoon the bananas and sauce over vanilla ice cream in bowls and top with toasted walnuts or top the whole pan with 3-4 scoops of ice cream and toasted walnuts.