Step 1Heat the oven to 500 degrees. Lightly oil a 9-by-12-inch baking dish. Pour 3 tablespoons of the olive oil into one corner of the dish. Season this puddle with 1 teaspoon salt and one-fourth teaspoon pepper (you'll use it to dip the potatoes).
Step 2Halve the potatoes. Dip each half into the seasoned olive oil, then put the halves back together with one bay leaf sandwiched between, leaving the prepared potatoes in the dish.
Step 3Cut the onions into 8 lengthwise wedges. Separate the layers of the fennel bulb and cut the pieces into 1-inch strips.
Step 4Arrange the vegetables in the dish, alternating seasoned potatoes with onion and fennel pieces. Drizzle with the remaining olive oil. Season with 1 teaspoon salt and one-fourth teaspoon pepper. Top with the sprigs of fennel, and cover lightly with foil.
Step 5Place the dish on the middle rack of the hot oven and reduce the heat to 450 degrees. Cook for 30 to 45 minutes, until the potatoes are almost tender. Remove the foil and continue to roast an additional 15 minutes.
Step 6Remove the dish from the oven and turn the potatoes, onions and fennel so the other sides caramelize. Return to the oven and roast another 10 to 15 minutes, until the bay leaves are charred and brittle, the onions brown and molten, and the fennel pieces papery and crunchy or molten. Serve immediately.