In a large pot over medium heat, melt the butter. Add onion and cook until softened (3-5 minutes).
Add broth and cauliflower, bring to a boil. Reduce heat slightly and let it simmer until the cauliflower is tender (20-30 minutes).
Meanwhile, grate the Cheddar cheese and whisk milk and flour together in a bowl.
Once cauliflower is tender, slowly add the milk-flour mixture, stirring until combined. Simmer until soup thickens (about 5 minutes).
Off heat, stir in Cheddar until melted and well mixed. Season with salt and pepper to taste.
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