Preheat the oven to 400F. Toss the cauliflower, red onion, salt, paprika, and crushed red pepper in 1 1/2 tablespoons of the oil. Transfer the vegetables to a baking sheet lined with parchment paper or foil. The florets should be in a single layer and now crowded; if you need to use two sheets to achieve this, that's fine. Roast for 25 minutes, or until the cauliflower is tender and crispy on the edges, stirring once halfway through.