Place a heavy-bottomed pot over medium-low heat. Add the butter and allow it to melt completely without browning.
Sprinkle in the all-purpose flour and stir continuously until the butter and flour form a smooth mixture. Keep the mixture pale rather than allowing it to brown.
Gradually pour in the whole milk, stirring constantly to prevent lumps. Add the half-and-half and continue stirring until the soup base becomes smooth and creamy.
Stir in the whole kernel corn and chopped green onions. Cook for a few minutes, stirring occasionally, until the vegetables are tender.
Gently fold in the crabmeat, white pepper, seasoning salt, and soy sauce. Stir carefully to distribute the ingredients without breaking the crabmeat into very small pieces.
Reduce the heat if necessary and allow the soup to simmer gently until heated through. Small bubbles should begin forming around the edges, but the soup should not come to a full boil.
Taste the soup and adjust the seasonings if needed.
Ladle the hot soup into individual bowls, garnish with freshly chopped parsley, and serve immediately.