With the rack in the middle position, preheat the oven to 200°C (400°F). Line a baking sheet with parchment paper.
Place the pancetta slices on the baking sheet. Bake until crisp, about 15 minutes. Drain on paper towels. Set aside.
Make the mayonnaise: In a bowl, whisk together the mustard, honey, garlic and egg yolk. Whisking constantly, add the oil in a thin stream. Season with salt and pepper.
For each sandwich, spread 1 bread slice with mayonnaise. Top with 2 slices of pancetta and tomato. Season with pepper. Top with 1 bread slice spread with mayonnaise. Top with duck and arugula and finish with 1 bread slice. Secure with 4 toothpicks and cut into quarters.
If you have time to make them, serve with Root Vegetable Fries.