2 large Haas avocados (about 1 lb.)
1 Tbs. fresh lemon juice, more to taste
1 large tomato (about 3/4 lb.), 8 slices
2 Tbs. olive oil
8 thick slices bacon (about 10 oz.)
1 large Vidalia onion, cut into 1/2-inch-thick rounds (about 8 rounds)
Eight 1/2-inch-thick slices bread (rustic white, whole wheat,or sourdough), toasted
8 Boston lettuce leaves
In a small bowl, mash one of the avocados with the lemon juice and 1/4 tsp. salt. Taste and season the avocado with more lemon juice and salt if needed. Slice the second avocado into thin slices. Fan the tomato slices on a large plate, drizzle with 1 Tbs. oil, and sprinkle with 1/2 tsp. salt.
In a large (12-inch), heavy-duty skillet (like cast-iron), cook the bacon with the remaining 1 Tbs. oil over medium heat, flipping occasionally, until it browns and renders much of its fat, about 5 minutes. Transfer the bacon to a plate lined with paper towels. Add the onion rounds to the skillet, sprinkle lightly with salt, and cook until they start to brown, about 2 minutes.
Flip and cook until the other sides brown but they are still firm, about 2 more minutes. Remove from the heat.
For each sandwich, spread the mashed avocado on one bread slice. Top with 2 slices of tomato, 2 onion rounds, bacon, lettuce, and avocado slices. Add the remaining slice of bread, and serve.