Crank the oven to 350°F.
Place the bacon on a rimmed half-sheet panA lip is important as there will be a fair amount of fat.. Roast, flipping halfway through, until crisp, 20 to 25 minutes. Carefully remove pan from oven and transfer the bacon slices to paper towels to drain. Carefully pour the bacon fat from the sheet pan into a small heatproof bowl and reserve both the fat and the sheet pan. Keep the oven on.
While the bacon is making the kitchen smell awesome, trim the dry ends from the sprouts, remove any yellow outer leaves, and halve the sprouts longitudinally. Cut the apple by quartering, then remove and discard the core. Slice each quarter lengthwise into thirds, then cut crosswise into fourths to yield 12 pieces. Do not peel the apple!
In a large bowl, toss the sprouts and apple pieces with 2 tablespoons of the reserved bacon fat and spread in an even layer on the reserved sheet pan, placing as many sprouts cut-side down as possible. Reserve the bowl.
Roast until the sprouts are brownedA little char is a good thing. and tender, 20 to 25 minutes.
While the sprouts and apple cook, whisk the mustard and syrup together in the reserved large bowl. Coarsely chop the bacon.
When the sprouts are brown, return them to the bowl, and toss with the mustard-maple mixture. Plate and top with the bacon.