Preheat the oven to 450°F. Line a half-sheet pan with parchment paper for easier clean-up, though it is not essential. If you have a convection setting on your oven, Leahy "highly" encourages you to use it when roasting the cauliflower because it will caramelize faster and more evenly. Set the oven at 425°F convection and monitor the final minutes for browning, since the cauliflower will brown faster this way.
If any of the cauliflower florets are quite large, halve them so the pieces are similar in size. Place them in a large bowl; add the 3 tbsp. oil, the kosher salt, cumin, and coriander (if using); and thoroughly mix together using your hands.
Spread the cauliflower out in an even layer on the prepared pan. Roast until the cauliflower starts to soften and has browned around the edges, about 15 minutes.
Meanwhile, in a small bowl, mix together the onion, raisins, vinegar, and remaining 1 teaspoon oil.
After the cauliflower has roasted for 15 minutes, remove the pan from the oven and add the onion-raisin mixture, stirring it into the cauliflower. Roast until the raisins are a little crispy and the onion and cauliflower are tender and slightly charred in places, 6 to 10 minutes more.
Sprinkle the cilantro over the top and season with flaky sea salt, if desired. Serve immediately.