Preheat the oven to 400ºF. Remove the leaves and stem from the cauliflower and cut it into bite-sized florets. Place the florets in a colander and rinse under cool water. Drain well and remove as much water as possible.
While the cauliflower is draining, slice the onion into strips. Line a baking sheet with foil, if desired, for easy cleanup. Spread the cauliflower and red onion on the baking sheet. Drizzle with olive oil and sprinkle on the seasonings (curry powder, red pepper flakes, salt, and pepper). Toss until everything is well coated.
Place the baking sheet in the oven and roast for 45 minutes, stirring once half way through. Thaw 1/2 cup of frozen peas while the cauliflower is roasting.
Remove the cauliflower from the oven after 45 minutes or when the edges of the cauliflower have turned golden brown. Add the thawed peas and toss until everything is coated in oil and spices. Serve warm!