Melt butter in a large pot over medium-high heat. Add onion and sauté for 3-5 minutes, or until tender. Add garlic and cook an additional minute.

Sprinkle flour on top and cook for 1-2 minutes, or until light brown in color. Whisk in milk, chicken stock and salt until well combined and bring to a boil.

Add cauliflower, reduce heat to medium, and let simmer for 15 minutes or until cauliflower is fork tender. Pour soup into a blender (or use an immersion blender) and blend until smooth then add back to the pot.

Mix in greek yogurt, shredded cheeses and cooked chicken until combined and warmed through.