Preheat the oven to 180°C.
Rub the butter and flour together in a bowl until they reach a breadcrumb texture.
Mix in the egg and salt, wrap in cling film, and chill for 10 minutes.
Roll the pastry to fit a 18cm tart tin and chill for another 10 minutes.
Blind bake the pastry shell with weights for 15 minutes, then for 10 minutes without weights until dry.
Fry the bacon, garlic, and onion in butter until softened and golden-brown.
Whisk the cream, eggs, and egg yolks together in a bowl.
Layer bacon mixture and cheese in the case, pour the eggs over, and top with more cheese.
Bake for 30 minutes until the filling is just set.