Preheat the oven to 400ºF and set the rack in the middle.
Toss the chickpeas and cauliflower together in a large roasting pan with 3 tablespoons of olive oil and a big pinch of salt.
Roast, stirring now and then, until everything is dark brown and the cauliflower is quite soft, about 45 minutes.
Meanwhile, whisk together the mustards, vinegar, and ¼ cup of olive oil with a big pinch of salt and a few healthy grinds of black pepper.
While the chickpeas and cauliflower are still warm, toss them with the mustard dressing and the parsley. Serve warm or at room temperature.