Season both sides of each chicken fillet with freshly ground black pepper. Brush both sides with roughly 1 tablespoon of grainy mustard.
Arrange 3 bacon slices in overlapping rows on a work surface. Press flat with a wide knife. Wrap one chicken fillet tightly in the bacon. Repeat with the remaining 3 fillets.
Place wrapped fillets in a baking dish. Bake at 190°C (374°F) for 25-30 minutes until the internal temperature reaches 74°C (165°F).
Sprinkle with finely chopped fresh chives and serve immediately.