Heat the oven to 425 degrees. Place the cauliflower florets and chickpeas onto separate sheet pans and drizzle with olive oil. Season with salt and pepper. Roast chickpeas for 15 minutes then remove from oven. At the same time roast cauliflower for 25 minutes, until the cauliflower is golden and tender.
While cauliflower and chickpeas roast, heat a medium skillet to medium-high. Add 1 tablespoon of olive oil and the onion, and sauté until soft and fragrant, 1-2 minutes. Add the garlic and cook for 1 minute longer, stirring constantly to keep from scorching. Pour the stock into the pan and simmer until reduced by half, about 5 minutes. Reduce heat to low, then stir in the butter, capers, orange zest and juice. Season with a few turns of black pepper.
To serve, combine the cauliflower and chickpeas in a large bowl. Top with the sauce and a sprinkling of parsley.