Combine olive oil and cumin in a large bowl. Add the cauliflower florets and toss to combine. Transfer to the baking sheet; placed flat side down. Season with a few good pinches of salt and pepper, to taste. Place in the preheated oven and roast for about 20-25 minutes, or until tender and lightly browned, depending on size of the florets. Turn the pan and toss the cauliflower halfway through for even cooking.