Make the chicken bacon ranch dressing base. In a large mixing bowl, stir together the mayonnaise, sour cream, prepared ranch dressing, dry ranch seasoning mix, lemon juice, garlic powder, onion powder, black pepper, and salt. Whisk or stir until the mixture is smooth and creamy. Taste and adjust seasoning if needed, keeping in mind the bacon and cheese will add some salt.
Add the mix-ins. To the bowl with the dressing, add the chopped chicken, most of the crumbled bacon (remember you’ve reserved some for topping), diced celery, chopped green onion or chives, and shredded cheddar cheese if using. Fold gently with a spatula or big spoon until everything is evenly coated and you see a nice chunky, creamy filling.
Chill the filling so the flavors blend. Cover the bowl and refrigerate for at least 30 minutes, or up to 24 hours. This resting time helps the ranch flavors soak into the chicken and lets the mixture firm up a bit, which makes it easier to scoop into lettuce later. If it looks a little thick after chilling, you can loosen it with a spoonful or two of extra ranch dressing.
Prepare the lettuce “sandwich” layers. While the filling chills, separate the romaine, butter, and/or iceberg leaves. Rinse if needed, then pat very dry with clean towels so the filling doesn’t slide around. Choose sturdy leaves about hand-sized for the bottoms, and slightly smaller leaves to lay on top like the other “slice” of bread. If packing for a potluck, line 3 clean bakery boxes or shallow containers with a layer of large lettuce leaves to help keep everything snug.
Assemble the lettuce sandwiches. Spoon a generous scoop (about 1/4 to 1/3 cup) of the chilled chicken bacon ranch filling onto a lettuce leaf. Spread it gently to cover the center, leaving a little border so it doesn’t spill out. Lay a second lettuce leaf on top, pressing lightly to create a neat little bundle. Repeat with the remaining filling and lettuce, arranging the sandwiches snugly in rows in the bakery boxes or on a platter.
Finish with bacon bits and garnish. Once all the lettuce sandwiches are nestled in the boxes, drizzle a thin ribbon of extra ranch dressing over the top if you like. Sprinkle the reserved bacon crumbles and chopped parsley or dill over everything so you see bits of bacon and green herbs on top. This makes the boxes look full and inviting when you set them out on the counter.
Chill until serving. Cover the bakery boxes or containers and refrigerate until you’re ready to head to the potluck or put them on the table. They hold well in the refrigerator for several hours. At serving time, set the boxes on a marbled or light-colored countertop, open the lids, and let folks help themselves. Expect people to start with one, then circle back for a second (or third) once they get a taste.
