Steam head of cauliflower on the stove or in the microwave. (To microwave: add the florets to a large glass dish with enough room so that there is one inch between cauliflower and rim, add 1 inch of water and cover with saran wrap. Place in microwave for 10-13 minutes or until tender/easily pierced with a fork.)
Preheat oven to broil.
Add all ingredients (except shredded cheddar) to the food processor or Vitamix (leave the lid off to allow venting if florets are still hot) and process until desired consistency is reached.
Add mixture to an oven safe dish and top with cheddar cheese and a sprinkle of chives. Broil for 2-3 minutes until cheese is melted, bubbly, and golden brown.
To Make Ahead In the Oven:
Follow the instructions for 1 & 3 but do not turn on the oven. Once the cauliflower mixture is ready, place it in an oven safe dish that has a cover, such as a pyrex container. Top with shredded cheddar cheese, cover, and store in the refrigerator.
When you are ready to reheat the cauliflower mixture, preheat the oven to 350F. Cover the dish with foil and bake for 35-45 minutes until hot in the center. Uncover dish and broil for 1-3 minutes until cheese is melted, bubbly, and golden brown.
To Make Ahead Using The Microwave:
Alternatively, you can follow the follow the instructions for 1 & 3 but do not turn on the oven. Once the cauliflower mash is done place it in a microwave-safe dish that has a cover, such as a pyrex container and then place it int he refrigerator. DO NOT TOP IT WITH CHEESE.
When you are ready to cook it, remove the dish from the refrigerator. Microwave for 10-15 minutes stirring every five minutes until the dish is hot in the center. You can then add the cheese on top. Melt the cheese using the microwave or place the whole dish under the broiler for 1-3 minutes until cheese is melted, bubbly, and golden brown. (This can be done in the toaster oven as well!)