Preheat oven to 450°. Line a rimmed baking sheet with heavy-duty aluminum foil.
In a large skillet, cook bacon over medium-high heat until crispy. Using a slotted spoon, remove bacon from pan, leaving as much grease in the pan as possible. Set bacon aside on paper towels.
In a large bowl, combine potatoes, fennel and onion. Pour reserved bacon grease over potato mixture, tossing gently to coat. Add salt and pepper, to taste. Spoon mixture in an even layer onto prepared baking sheet. Roast for 40 minutes. Stir occasionally or until potatoes are browned and tender. Add reserved bacon, tossing gently to combine.