Set the 14" x 14" piece of parchment paper flat on your work surface. The parchment should sit smooth and open so you can use it to guide and tighten the roll later.
Arrange 5 to 8 large lettuce leaves in the center, overlapping them until you have a base about 9" by 10". If a few leaves crack, patch the gaps with smaller pieces so the filling has a solid, gap-free layer underneath.
Spread the grainy mustard across the middle of the lettuce base. Keep it centered so the edges stay cleaner and the wrap closes more easily.
Layer the turkey, cheese, bacon, tomato, and red onion over the mustard. Stack the fillings in the middle rather than spreading them too wide so the wrap stays compact.
Lift the edge of the parchment closest to you and start rolling the lettuce forward over the filling. Pull gently but firmly as you go so the roll begins tight instead of loose.
Tuck both sides inward once the wrap starts to take shape. This burrito-style fold helps keep the filling from slipping out the ends.
Keep rolling forward, using the parchment to press and shape the sandwich into a firm cylinder. The wrap should feel compact and secure in your hands before you stop rolling.
Wrap the remaining parchment around the outside of the lettuce roll. A snug paper wrap helps the sandwich hold its shape and makes it easier to handle.
Cut through the wrap with a serrated knife, stopping just before you slice through the bottom layer of parchment. Leaving that last bit of paper attached helps the sandwich stay together while you eat it.