Peel 5 pounds potatoes and cut into small dice. Cover with 3 quarts water, add 2 Tablespoons salt and 1 Tablespoon monosodium glutamate, and cook, covered, until the potatoes are mushy. Whisk or beat until the mixture forms a puree.
Meanwhile, chop 3 pounds onions. Melt 1/2 pound butter or margarine in a heavy pan, add the chopped onions, and cook, stirring often, until golden. (Do not allow the onions to brown.) Sprinkle with 1/2 cup flour and cook, stirring, for a few minutes.
Heat 4 quarts milk and add gradually to the onion mixture, stirring. Cook, stirring, until the sauce is smooth and thickened. Simmer 10 minutes, stirring often. Combine the sauce with the potato puree. Heat well.
Add 2 cups cream and heat gently (or use all milk or undiluted evaporated milk). Taste and adjust the seasoning with more salt and a little white pepper. Finely chop fresh chives or scallion greens to make about 1 cup. Serve the soup with a ladle, about 3/4 cup to a portion, and sprinkle each serving with about 1/2 teaspoon chopped chives. Serves 50 or more.