Get the white sauce ready. Melt butter in a pot on medium-low, then stir in flour and 1/4 teaspoon salt until combined. On medium heat, add the milk a bit at a time. Whisk the mixture constantly until it thickens and is smooth. On medium heat, add onions to a 2-quart saucepan with the butter.

Cook the onions until they are translucent. Reduce the heat, add the water, add the spinach, and cover the pan. The spinach should be stirred numerous times until nearly done. Salt is added with 1 teaspoon. Add the white sauce and sour cream when the spinach is almost done cooking. Stir well, then boil until well combined.