Melt butter in a heavy saucepan. Over medium-high heat, sauté the onions, carrots and celery until translucent and tender, about 3 to 5 minutes. Stir in flour and cornstarch. Cook about 3 to 5 more minutes. Add stock and milk gradually, blending until smooth, and reduce by 1/4. Do not allow to boil at any point. Add baking soda and cheese and stir until melted and thickened, about 10 minutes.
Add parsley, cayenne, and paprika. Keep soup warm over very low heat or in a double boiler if not using immediately. Season with salt and pepper to taste.Makes 6 to 8 Source: