Make Homemade Mayo: Whisk together egg yolks, mustard, salt, and vinegar. Trickle feed in oil while whisking until the mixture thickens to a butter-like consistency. Add a tiny bit of lemon juice.
Prepare the Bacon: Fry the bacon in a pan, adding a little extra oil if needed. Remove bacon when crispy, keeping the fat in the pan.
Marinate Tomatoes and Shallot: Slice the tomato and shallot. Marinate in cherry vinegar, rapeseed oil, salt, and pepper.
Prepare the Bread: Cut three slices of potato bread. Fry one slice in the bacon fat until crispy for the sandwich's middle layer.
Build the Sandwich: Spread mayo on the bottom slice of bread. Add marinated tomatoes and shallots, lettuce, the fried bread slice, more mayo, lettuce, and bacon. Top with the final slice of bread. Press down to compact the sandwich.
Serve: Cut and serve the sandwich immediately.