Step 1

In a bowl, toss onion with vinegar and 1/4 tsp each salt and pepper; let sit, tossing occasionally.

Step 2

Cook bacon in the oven or on the stovetop until crisp; transfer to a paper towel–lined plate.

Step 3

To the bowl with the onion, add tomatoes and oil and toss to combine. Fill lettuce leaves with bacon and tomato mixture and top with blue cheese if desired.