Combine the water, bouillon cubes and salt in 5-quart crock pot and turn on high. Cut the white onion in half and coarsely chop the half onion, half carrot, and half celery stalk. Add to the crock pot. Put the lid on and let it cook for 4-5 hours. When the broth is done, strain the veggies out of the broth. Adjust seasoning to taste, and set the crock pot to “warm”.
Add the oil to a small skillet or sauce pan and very thinly slice the other half of the white onion, separating the pieces. Dip in the milk and then toss in the flour until coated. Fry a handful at a time in the oil till golden brown, then drain on a paper towel.
Ladle 1 cup of broth into a bowl. Sprinkle a few fried onion strips into each bowl of soup. Slice 1 mushroom for each bowl and sprinkle diced green onion. After the fried onion strips fall to the bottom of the bowl (it will take just a few minutes), serve the soup.
For Faster Version: Use 4 cups chicken broth and 2 cup water instead of bouillon cubes.