Dice or slice the bacon, pancetta or lardon.
On medium heat render out the fat for about 10 minutes. Move the pieces around to avoid burning down and uneven browning.
We are going to use the rendered out fat to caramelize the onions. Add the onions and saute it on high heat for about 5 -10 minutes.
Add garlic and scallion then on medium heat fry it until the garlic gets fragrant for about 2 – 5 minutes. This is the time to add optional vegetables like spinach too. It can be added frozen, in which case at least 5 -10 more minutes of frying recommended.
Beat eggs then mix in the cream and milk. If we run out of milk or cream just substitute one with the other.
Pour the eggs mixture into the pan and add the grated cheese too.
Mix everything together then fry the fritatta on medium heat until the outer rim sets slightly. Don't fry it on high heat! We can also just put the whole thing into the oven now as we do with crustless quiche.
Place the fritatta into the middle rack of a 400°F / 200°C preheated oven and bake it until the middle sets for about 10 minutes. If we are after some fancy look, wait for the golden brown spots too.