Preheat oven to 400 degrees and place cauliflower on a large baking sheet. Drizzle cauliflower florets with oil and season with salt. Roast cauliflower florets about 20 minutes until tender. Transfer to a colander and drain as much liquid as possible.

Place a saucepan over medium heat and melt butter. Add flour and whisk to combine. Allow flour to cook for about 30 seconds before whisking in milk, salt and garlic powder. Bring to a boil then reduce heat to low and simmer for 5 minutes.

Remove from heat and whisk in Monterey jack cheese. When cheese is melted, stir in sour cream then season to taste with salt and set aside.

In a large bowl, combine drained cauliflower, sliced green onion, crumbled bacon and cheese sauce. Stir to combine, test for seasoning and transfer to a 2-qt casserole dish that has been lightly sprayed with cooking spray.

Sprinkle top with cheddar cheese and bake 25–30 minutes until bubbly. Remove from oven and allow to rest for at least 5 minutes before serving.