Steam half a head of cauliflower until soft when poked with a fork (about 5 to 8 minutes). Drain the cauliflower and set aside to cool.
Over medium heat, sauté the sliced onion in a couple teaspoons of oil until fragrant and beginning to turn translucent, about 5 minutes.
Once the cauliflower is cool, combine the steamed cauliflower, milk, flour, salt, nutmeg and thyme in a blender. Blend until combined
Crack the egg into the cauliflower/milk mixture and blend just enough to combine (this should make just over 3 cups worth of mixture).
Preheat the oven to 400 degrees.
Cover the casserole dish with foil and bake for 50 minutes. Remove the casserole dish from the oven, discard the foil.
Put the casserole dish back in the oven (uncovered) and bake an additional 15 to 20 minutes or until golden brown. Allow the dish to sit 10 minutes so before you serve.