Make a rich custard and precook the filling ingredients
For the custard, I use only cream and fresh eggs. Some cooks try to lighten the mix by using milk or even (horrors) low-fat milk, but this is a mistake. The whole point of a tart or quiche is the rich, binding nature of the custard. Skimp on the custard and you’ll get a watery, sad-looking tart.
I generally blend the cream and eggs by hand with a whisk. Once they’re well combined, I add a pinch of salt, a few grinds of pepper, and any other herbs or spices (a pinch of nutmeg or cayenne is a good addition). You can make the custard hours ahead, as long as you cover and refrigerate it, but don’t hold it for long once the filling ingredients are added to it.
Cheese is a natural marriage with custard, and here you can be creative. Goat cheese, blue cheese (especially the mild Gorgonzola dolce), and Stilton are wonderful with custards, as are Gruyère and Comté. I also like to use fresh herbs like parsley, thyme, sage, chervil, and sometimes a bit of rosemary. Keep in mind that milder herbs, like parsley, chervil, and dill—lose some of their oomph when cooked, so use generous amounts.
The most successful quiches or tarts are made with members of the allium family: onions, leeks, chives, or scallions. I find it’s best to precook these, and most other filling ingredients, so that they don’t leak moisture into the tart. Sauté leeks, shallots, and onions in butter and then cool and drain them well. Be sure to cook meats like sausage and bacon and drain them of all fat.
Once all the filling ingredients are prepared, carefully ladle the custard and filling into the shell. Be careful not to overfill, as the custard will puff, and if the custard seeps over the sides, you’ll have a heck of a time getting the tart unmolded.