Heat tablespoon oil in a large frying pan over medium heat. Add onions and cook until transparent. Add cauliflower and broth and bring to a boil. Stir occasionally for 8 minutes.
Remove pan from the heat. Remove two cups of cauliflower and add to the blender with milk. Blend until smooth. Add cheddar, parmesan, sea salt and lemon juice and blend again until cheese has melted.
Add spinach to the cauliflower rice and put the pan back on heat.
