Roasted Cauliflower with Onions and Fennel

Name: Roasted Cauliflower with Onions and Fennel
Ingredients: 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