1. Preheat your oven to 400ºF.
2. Place 1 inch of water in a large pot with 1 teaspoon salt and bring to a boil. 
3. Add in the cauliflower and cook until tender (about 8 minutes)
4. Drain and set aside in a large bowl.
5. Heat the butter and oil in a large saucepan over medium heat. 
6. Add onions and cook them for about 2.5 minutes.
7. Stir in the flour, reduce heat to low and cook while stirring the mixture for about 5 minutes to cook the flour.
8. Raise the heat to medium-high and add in the milk while you whisk the mixture until it comes to a boil for about 2.5 minutes 
9. Once the mixture thickens, remove the mixture from the heat completely. 
10. Slowly add the cheddar cheese until all the cheese is melted.
11. Stir in the cooked cauliflower. 
12. Pour the mixture into a baking dish and bake until golden.
13. Broil for 1-2 minutes, until the top is browned.