Remove leaves and lower portion of the stalk from the head of cauliflower. You can keep a portion of the stalk to roast. You’re going to puree it up anyways! I like to trim only 1-1 1/2″ off the bottom, at most.
Chop cauliflower into florets. Try to keep them all the same size so that they roast equally. I chop them a little larger than bite-sized. If you want them to roast quicker, cut them smaller.
With a paper towel, pat the cauliflower pieces to make sure they are fairly dry. This helps them to roast instead of steam in the oven.
Toss the cauliflower pieces in a large mixing bowl with oil to coat.
Spread cauliflower onto a large baking pan lined with parchment paper. (easy clean up!) Make sure they are spread out and not touching much.
Bake for 40-50 minutes or until the cauliflower is very tender. It’s ok if some pieces are a little overdone. That will give the mash flavor!
To a food processor or high-powdered blender, add the cauliflower pieces, butter, milk, salt, garlic powder, and onion powder. Blend until smooth. I start with 2 tbsp of milk and 1 tsp of salt and adjust the taste and consistency after that.
Transfer the mash to a bowl and mix in fresh chives or finely chopped green onions, if desired!