Place bacon in a large skillet; cook and stir over medium-high heat until evenly browned and crispy, about 10 minutes. Add lettuce; cook and stir in the bacon grease until lettuce wilts, 2 to 3 minutes. Mix buttermilk, salt, and pepper into bacon-lettuce mixture; cook and stir until buttermilk sauce thickens, 5 to 10 minutes. Whisk flour into sauce if needed to thicken.
