Place a baking wire rack on a sheet pan and arrange the bacon on the rack in a single layer. Bake for 15 to 25 minutes, until the bacon is browned and crisp. Drain on paper towels and set aside.
Place the bread slices on a sheet pan and toast them for 5 minutes. Turn the slices and toast for 1 more minute, until all the bread is evenly browned. Place 2 slices on a cutting board. Spread each slice with about 1 tablespoon of mayonnaise. Cover the slices with a layer of lettuce. Peel the avocado and slice it 1/2-inch thick. Toss the avocado slices gently with the lemon juice, and arrange on top of the lettuce leaves. Place 3 slices of bacon on top of each sandwich and then add a layer of tomato slices on top of the bacon. Sprinkle liberally with salt and pepper.
Spread each of the remaining 2 slices of toasted bread with 1 tablespoon of mayonnaise and place, mayonnaise side down, on the sandwiches. Cut in half and serve.