Boil eggs, peel, and mash with 2–4 tbsp mayo (adjust to your liking), salt, pepper, and bacon juice.

Air-fry bacon at 200°C for 20 min, turning halfway. Reserve bacon juice.

Finely chop half a red onion.

Spread mayo and butter on each bread slice.

Spread egg mixture on 2 slices, top with bacon and onion, then cover with remaining bread.