Preheat oven to 425℉/218℃. Brush a large baking sheet with a thin layer of olive oil and set aside.
Prepare the cauliflower by cutting the head into medium florets of roughly the same size.
Add a couple inches of water to a pot and bring to a boil over high heat. Place a vegetable steamer basket in the pot and add the cauliflower florets. Cover and steam for 5 to 7 minutes until the cauliflower can be easily pierced through with a fork but is not falling apart. Do not overcook. Remove steamer basket from pot.See notes for alternative cooking methods and tips when using frozen cauliflower.
Transfer the florets to the prepared baking sheet and arrange in a single layer.
Use the bottom of a drinking glass or glass jar to smash each floret to flatten it.
In a small bowl, combine the rest of the olive oil, garlic and onion powders, thyme, and salt. Brush this mixture on top of each floret. Sprinkle the parmesan cheese on the florets.
Transfer the cauliflower to the center rack of the preheated oven. Bake for 20 to 25 minutes until crispy and lightly browned on the outside but tender on the inside.
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Notes
Baking sheet tip: You can line the baking sheet with parchment paper before brushing with olive oil for easier clean-up, but I find that skipping parchment and placing the florets directly on the pan makes them crispier.
Frozen cauliflower tip: If using frozen cauliflower, decrease the steaming time by a few minutes and check for doneness before 5 minutes. It usually takes less time to cook than fresh since it is blanched before being frozen.
Boiling vs. steaming: If you don't have a steamer basket, use a fine mesh strainer. If you don't have either, you can boil the cauliflower. However, it may retain water and therefore not get as crispy in the oven. If boiling is the only option, cook the cauliflower in a pot of water or skillet with water for only 3 to 4 minutes. Drain well and pat dry before smashing.
Leftovers and reheating: Store leftovers in an airtight container in the fridge for 2 to 3 days. It will lose some of its crispy texture over time, but you can get some of it back by reheating in a skillet or oven. Add a little oil to a skillet and cook for a few minutes on each side over medium heat. For the oven method, preheat the oven to 400 degrees F/204 degrees C. Place the florets on a baking sheet and cook for 5 to 10 minutes until warm.