Preheat oven to 400 degrees F. Toast the fennel seeds in a skillet over medium heat for 3 to 5 minutes, shaking the pan often to prevent seeds from burning. (Toasting releases natural oils in the seeds.) Place potatoes, onion, and fennel wedges in a large bowl. Add the toasted fennel seed, rosemary springs, olive oil, and a few pinches of salt and pepper; toss.
Transfer the vegetable mixture to 1 large or 2 small parchment-lined rimmed baking sheets. Roast 35 to 45 minutes or until vegetables are crispy on the outside and tender on the inside when pierced with a fork. Remove from the oven, toss with the chopped fennel tops, and serve warm. Makes 8 to 10 servings.