Cook bacon in a cold skillet over medium heat, 8–10 minutes, until crisp.
Cool bacon on paper towels 5 minutes, then chop.
Cook chicken breasts to 165°F internal temperature (or use pre-cooked/rotisserie chicken); cool 10 minutes.
Shred or dice the cooled chicken.
Dice tomatoes and drain on paper towels 5 minutes.
Chop lettuce into thin ribbons.
Whisk mayonnaise, Dijon mustard, and lemon juice until smooth.
Season dressing with garlic powder, salt, and pepper.
Fold chicken into dressing.
Add bacon and drained tomatoes; fold to combine.
Chill 30 minutes.
Fold in lettuce just before serving.