Melt ½ stick butter in dutch oven or heavy pot on medium heat and add onion. Cook for a few minutes until onion is translucent.
Add carrots and celery and cook for a few more minutes. Add cauliflower and parsley, stir to combine.
Cook for 15 minutes over low heat. Add stock and bring to boil and then lower to simmer for around 10 minutes.
Meanwhile melt the other ½ stick of butter in sauce pan and whisk in flour. Cook for a few minutes. Stir in milk and allow to heat up for a few minutes. Take off heat and add half & half.
Add milky mixture to simmering soup and check seasoning (I used bout 2 tsp of salt and 1 tsp of pepper). Allow to simmer for 15 to 20 minutes.
Drop 1 to 2 tbsp of yogurt or sour cream into each bowl. (Totally optional) Ladle soup over top and enjoy!
Recipe by pretty plain janes at https://prettyplainjanes.com/2015/01/08/cauliflower-soup/