Broccoli Cheese Soup

I have been making this soup for over 35 years and still read it from my submission to the Laurel Mountain Elementary cookbook from '87-'88!

1/4 c. butter
1/2 c. diced onion*
1/2 c. diced carrot*
1/2 c. finely diced celery*
1/4 c. flour
1  1/2 Tbsp. cornstarch
1/8 tsp. baking soda
4 c. milk, (room temperature)
4 c. chicken stock, room temperature
1 lb. processed cheese (Old English or Velveeta) diced
1 tsp. salt
1 Tbsp. dried parsley
dash cayenne pepper, optional paprika
3 c. chopped, cooked broccoli

Melt butter in a heavy saucepan. Saute vegetables until tender. Stir in flour and cornstarch.  Cook until bubbly.  Add stock and milk gradually, blending into a smooth sauce.  Add soda and cheese pieces.  Stir until thickened and cheese is melted.  Add broccoli.  Season with salt and pepper.  Add parsley.  Before serving, heat thoroughly at low heat, DO NOT LET IT EVER BOIL.  It will curdle and not be smooth! Garnish with paprika.

*Note: I use a food processor for the onion, carrots, and celery. Add chunked vegetables and plenty of water to blender. Whir until veggies are minced. Drain and they're ready to sauté.