Preheat oven to 350 degrees. Grease two 9-inch round cake pans or 1 9x12 pan and line with parchment paper. Toast walnuts on a cookie sheet for about 10 minutes. Cool, then chop finely or pulse in food processor until chopped. Pieces should be the size of coarse bread crumbs, but don't process into a paste.
Combine flour, baking soda, salt and cinnamon in a medium-sized bowl. In a large mixing bowl, combine brown sugar and oil. Add eggs one at a time, beating well after each. Stir in flour mixture just until blended. Fold in grated carrot, mashed banana and nuts.
Pour into two prepared baking pans. Bake for 25-35 minutes until middle springs back when touched. Set aside to cool on racks for 10 minutes, then turn out and cool completely. Frost with Cream Cheese Frosting.