Preheat oven to 375 degrees. Coat a 9-by-11 pan with olive oil. Spread the onions evenly on the bottom of the pan and season them with salt and pepper. Layer potatoes on top and sprinkle with fennel seed. Season with salt and pepper to taste. Drizzle more olive oil and bake until the potatoes are almost tender. If desired, this is the time to sprinkle fresh rosemary (or even fresh thyme) on top of the potatoes if desired. Cook until the potatoes are fork-tender and lightly golden brown, about 30 minutes. Serves 4.