In small saucepan, melt butter over medium heat.
Add salt, pepper and onions and cook until tender.
Stir in flour to make a roux, cooking for about 1 minute.
Slowly stir in evaporated milk until mixture is smooth.
Pour sauce into slow cooker.
Add broth to slow cooker and stir until blended.
Add broccoli and stir.
Cook on low for 4 hours or high for 2 hours.
Add Velveeta cubes to soup and stir until melted.
Add cheddar cheese and stir until melted.
Add Parmesan cheese and stir until melted.
If soup is too thin, you can thicken with a cornstarch slurry(2 tablespoons cornstarch plus 2 tablespoons water well blended).
Garnish with a little cheddar cheese if desired.