In a small skillet, saute onion with salt in olive oil on low heat until golden brown.
In a 2-quart saucepan over medium heat, melt butter or margarine and stir in flour and pepper with a wire whisk. Stir constantly until foam subsides and flour begins to lightly brown.
While the flour is cooking, bring stock to a boil in a separate pan. Add boiling stock to cooked flour and stir until thickened. Reduce heat to low and cook for a few minutes.
Add onion, cashew butter and soy sauce to sauce and puree in a blender or food processor until smooth.