Preheat oven to 350 degrees F.
To a small skillet over medium heat, add walnuts. Cook until lightly toasted.
Together in a large mixing bowl, whisk flour, sugar, baking soda, and salt. Stir in walnuts.
Into a food processor, add bananas. Pulse 10 times, until mostly smooth but some chunkiness still remains.
Add vegetable oil, sour cream, lemon juice, and vanilla bean paste. Pulse 4 more times to combine.
Into the flour mixture, gently fold in the banana mixture, just until blended.
Into a prepared 9"x5" loaf pan, pour all the batter and smooth the top.
Bake 1 hour. After removing from oven, allow bread to stand in pan 10 minutes then remove from pan and transfer to a wire rack. Cool completely before slicing and serving.