Lay the bacon in a large skillet over medium heat and cook until crispy on both sides, flipping once, about 10 minutes. Remove the bacon to a plate and blot any excess grease with paper towels. Let cool slightly then cut each bacon strip into 4 equal pieces.
While the bacon is cooking, heat a griddle over medium-high heat. Add the baguette slices and cook, watching closely, until golden, 2 to 3 minutes on each side.
Spread each baguette slice with mayonnaise. Stack the toppings on each slice in the following order: lettuce leaf, tomato slice, bacon, and a sprinkle of scallions and sea salt.