Prepare the Sweet Mayonnaise: In a small bowl, whisk together the 1 tablespoon of grape jelly and the 4 tablespoons of mayonnaise until completely smooth and evenly combined. This unexpected mixture adds a sweet-tart, creamy element to the sandwich.
Toast the Rolls (Optional but Recommended): Lightly toast the interior cut sides of the diner rolls. This adds texture and prevents the bread from becoming soggy.
Assemble the Sandwich Rolls: Lay out the 8 bottom halves of the rolls. Spread the sweet mayonnaise mixture evenly across the bottom half of each roll. Place one slice of deli turkey on top of the mayonnaise. Follow this with one slice of the crispy turkey bacon.Distribute the sliced heirloom tomatoes and the thinly sliced red onion evenly across the 8 sandwiches. Add a small pile of the spring mix lettuce on top of the tomatoes and onion. Spread the 2 tablespoons of stone-ground mustard onto the top half of each diner roll. Season the filling lightly with freshly cracked black pepper. Cap each sandwich with the top half of the roll.
Serve immediately with a side of potato chips, coleslaw, or a dill pickle spear for the full diner experience.