Place all the ingredients together in a food processor and process until you have a smooth filling. If you aren’t using a food processor, chop the herbs first, then mash the ingredients together into a really fine paste. You don’t want a lumpy filling. You might try using a blender even.
In a 9-inch tart pan or a pie plate, lightly greased, place the crust dough and with your fingers pat it out to the sides, climbing up to the edges until you have an evenly patted-out crust.
Scrape the filling into it. Try to smooth it down as much as possible because this is a thick filling that is not going to settle on its own in the oven.
Bake in a preheated 350-degree oven for 45-50 minutes or until the filling is nicely set.
Cool for at least 10 minutes before sliding off the tart-pan sleeve.
To make the topping:
Heat the oil in a skillet. Add the onions and sugar and cook them, stirring frequently, on a medium-low flame until they are nicely golden-brown. This takes more than half an hour and some patience– you don’t want to hurry it up and burn them which would make them bitter. Scatter the onions on the quiche. Serve warm or at room temperature.