Preheat your oven to 250C (480F). Chop cauli florettes into bite sized chunks and slice stems. Chop onion in half lengthwise. Peel and finely slice each half into 1/2 moons. Place veg on a large rimmed baking sheet. Drizzle over 3 tablespoons olive oil and lots of salt. Toss to coat the cauli and onion in the oil. Pop the veg in the oven and set your timer for 15 minutes.
When the timer goes, stir veg and return the tray to the oven, setting the timer for 5 minutes.
When the timer is up, stir and check on everything. If the cauliflower is browned and no longer crunchy you can remove but if not, pop back in for another 5 minutes.