Candying the bacon in the oven adds a little bit of sweetness to the mix, enhancing a classic BLT. After the bacon gets crisp, you can overlap the strips, imitating the size and shape of your bread. As the bacon cools, the "caramel" on the outside firms up, making the sandwich easier to assemble -- and eat, too!
For the candied bacon: Preheat the oven to 375 degrees F. Line one rimmed baking sheet with aluminum foil and another with parchment paper. Arrange the bacon slices very close together on the foil-lined baking sheet -- they should all fit on one.
Sprinkle about 1 1/2 teaspoons brown sugar and a few grinds of black pepper over each slice of bacon. Bake until the bacon is evenly browned and very crisp, 15 to 17 minutes.
While the bacon is still hot, transfer to the parchment-lined baking sheet and divide into 2 groups, overlapping 5 slices slightly on top of each other. Let cool while you prepare the rest of the sandwich.
For the sandwich: Lay the tomato slices on a large plate and season with salt and pepper. Spread the mayonnaise on one side of each slice of bread. Heat a large cast-iron skillet over medium heat.
When the skillet is hot, add 2 slices of bread, mayo-side down, and toast until deeply golden brown. Remove to a plate and repeat with the remaining 2 slices of bread. Let cool completely. If desired, spread more mayonnaise on the untoasted sides of the bread.
To build each sandwich, layer half of the lettuce, basil, onion, tomato slices and bacon between 2 slices of bread. If desired, cut the sandwich in half.