Place a cast-iron skillet or heavy frying pan over medium heat. Arrange 4 bacon strips in a single layer. Cook, turning occasionally, until evenly browned and crisp, about 8–10 minutes depending on thickness. Transfer bacon to a paper-towel–lined plate to drain and keep warm. Reserve about 1 tsp of bacon fat in the pan for the egg (wipe excess if more).

Cook Egg

Fry the egg

Reduce heat to medium-low. Add about 1 tsp olive oil (or use reserved bacon fat) to the same skillet. Crack 1 large egg into the pan. For a runny yolk cook 2–3 minutes until whites are set; for a firmer yolk cook 3–4 minutes. Season egg with a pinch of kosher salt (about 1/8 tsp) and a small grind of black pepper. If you prefer, baste with a tablespoon of hot pan fat or cover briefly to set the top.

Toast Bread & Assemble

Toast the bread

Butter one side of each sourdough slice with about 1/2 tsp softened butter (total 1 tsp). Heat a clean skillet over medium heat and toast buttered sides down until golden, about 2 minutes. Alternatively, toast in a toaster and spread butter after toasting.

Spread mayonnaise

Turn the toasted bread slices so the untoasted sides face up. Spread 1/2 tablespoon mayonnaise on one slice and 1/2 tablespoon on the other (total 1 tbs).

Layer ingredients

On the mayo-spread side of one slice, place the lettuce leaf first (this protects the bread from sogginess), then lay tomato slices on top of the lettuce. Sprinkle the tomato with the remaining pinch of kosher salt (about 1/8 tsp) and a little black pepper. Add the 4 bacon strips in an even layer over the tomato. Place the fried egg on top of the bacon. If using, add avocado slices and/or pickles and a dash of hot sauce.

Finish & serve

Top with the second slice of bread, toasted side down. Press gently, slice the sandwich in half on the diagonal, and serve immediately while the egg is warm and the bacon is crisp.