In the morning: Heat a large skillet over medium. Add bacon, and cook until browned and crisp, 3 to 5 minutes per side. Transfer to a paper-towel-lined plate to cool. Place in a resealable plastic bag; store at room temperature.
Cut romaine into 1-inch ribbons; wash well, and dry. Place in a resealable plastic bag; refrigerate. Cut turkey, ham, and cheese into bite-size strips. Place in an airtight container; cover, and refrigerate.
Squeeze lemon into a small bowl. Mince scallion; add to bowl, along with sour cream, mayonnaise, and 2 tablespoons water. Season with salt and pepper. Whisk to combine. Cover and refrigerate.
At dinnertime: In a large bowl, combine lettuce, meats, and cheese. Add dressing, and toss to coat. Serve salad topped with crumbled bacon.