1 medium head of cauliflower (about 1 1/4 pounds), cored, cut into 1-inch florets
6 tablespoons olive oil, divided
2 medium onions (about 1/2 pound each), halved lengthwise, cut into 3/4-inch-wide wedges with some core still attached, peeled
2 fresh fennel bulbs (about 1 pound total), halved lengthwise, cut lengthwise into 1/2-inch-wide wedges with some core still attached
8 small garlic cloves, unpeeled
15 fresh marjoram sprigs (optional)
Adapted from Epicurious.com