Place a griddle or skillet over medium-low heat. Add bacon and place a bacon press, skillet, or masonry trowel on top of it to keep it flat as it cooks. Cook until lightly browned on first side, about 5 minutes, then flip, cover again, and continue cooking until bacon is browned on both sides and fat has rendered, about 3 minutes longer. Transfer bacon to a paper towel–lined plate and set aside.

Place bread on same skillet or griddle and toast in bacon fat over medium-low heat, swirling occasionally, until evenly browned on first side. Flip and brown second side.

Lay toasted bread on a work surface and spread mayonnaise on both top faces. Divide lettuce evenly between both pieces of bread. Layer tomato slices on 1 piece of bread and sprinkle generously with coarse salt and freshly ground pepper.

Break bacon slices in half and layer them onto the sandwich in 2 layers of 3 half slices each, alternating the orientation of bacon in each layer for more structural stability. Close sandwich and cut in half diagonally. Serve immediately.