Lightly grease a muffin tin (I used cooking spray) and preheat your oven to 180 C° / 350° F.
Heat a small non-stick pan over medium heat and fry the bacon for 1-2 minutes. Do not fry it too crispy, because it will get crispy in the oven. Take out and set aside.
Leave the bacon grease in the pan and add the sliced onions to it. Lower the heat on the pan and cook the onions slowly for 8-10 minutes until they are brown, soft and caramelized. Set aside.
In a small bowl whisk together 3 eggs, ¼ cup of milk and a good pinch of salt, pepper and nutmeg. Set aside.
If you do not have ultra fresh and soft white bread, I highly recommend to microwave the bread. That way it will be super soft and you will be able to mould it into the muffin tin. Otherwise it will crumble terribly. Just pop a couple of slices at a time in the microwave for 20-30 seconds.
Slice off the crust of the bread and roll each slice out thinly with a rolling pin. Butter one side of each slice and carefully stuff them into the muffin tin, buttered side on top.
Add cheddar, gorgonzola and mozarella to four of the cups and top them with the halved cherry tomatoes. The 3 cheese cups are done.
Add a teaspoon each of the cooked onions to the next four bread cups and divide the grated gruyere between them.
Line the next 4 cups with the bacon rashes.
Now quickly crack one egg into each of the bacon cups.
Fill the 4 onion cups with the egg/milk mixture.
Transfer into the oven and bake for 14-18 minutes or until the cheese has melted, the eggs have set and the cups are nice and brown.
To finish off, top the quiche cups with the remaining caramelized onions.