Preheat the oven to 400 degrees F (200 degrees C). Line a 12×18-inch baking sheet with foil or parchment paper.
Combine olive oil, garlic, Italian seasoning, salt, and freshly ground pepper in a small, microwave-safe bowl. Microwave on High for 20 to 30 seconds. Watch carefully, and remove from the microwave when the oil bubbles. Set aside.
Combine cauliflower florets, red onion, and red bell pepper in a large bowl. Pour in the olive oil mixture and stir until all the vegetables are coated. Spread out vegetables on the prepared baking sheet in a single layer.
Roast in the preheated oven until vegetables reach the desired doneness, 20 to 25 minutes. Broil if you like additional color, carefully watching that the vegetables do not burn, 1 to 2 minutes.
Sprinkle sliced olives over the vegetables and drizzle with balsamic vinegar. Garnish with fresh parsley. Serve warm.
This may also be served as a salad, with the vegetables at room temperature, tossed with chopped romaine lettuce.
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