In a 1 qt saucepan, melt butter & whisk in flour, cook on medium heat until the flour turns thick & comes away from the side of the saucepan. Pour milk in flour mixture a little at a time & stirring constantly, add salt. Mixture should thicken & become like thick pudding. Take off heat; set aside until ready to use.
NOTE: Remember to stir constantly taking care not to let the mixture lump.
2In a 2 qt saucepan, place 3 Tbsp butter & sliced onions. Cook over medium heat stirring frequently until soft & clear, but not brown. Add chicken broth from can, chicken bouillon, salt & pepper & stir ntil completely heated through. Add white sauce & velveeta cheese (white sauce will be thick because it has been removed from the heat). Simmer on medium-low heat until the cheese is melted & all ingredients are blended well, stirring constantly. Turn temperature to warm & let cook for additional 30 - 45 minutes.
3Sprinkle with shredded cheddar & bacon (if desired). Enjoy!