I have really dug into the art and science of the quiche since going vegetarian.  It's one of those foods I always thought was hard to make, until I tried making it.  I think the French name threw me off.  Which is funny, considering that quiche originated in medieval Germany, and its name "quiche" comes from the German word for "cake."

Your basic quiche is a 9 inch pie crust, filled with four eggs, and one cup of dairy. (I'm going to say "dairy" because most recipes specify "cream," but I always use the 2% milk I have on hand and it works out just fine.)  Beat the eggs with the milk, pour into the pie crust, and bake for about half an hour at 375 degrees.  (I like to bake the pie crust in the pan for ten minutes before adding the other ingredients, but this isn't mandatory.)

Of course, that would make a pretty sad and plain quiche!  From those base ingredients you can launch off and add just about anything.  As a general rule of thumb, the ingredients won't do much cooking once they are inside the quiche and being baked.  You will want to cook your additions first, if only by browning them in a pan with a bit of oil.

Personally I tend to use quiche in the same role as a casserole - as a kind of dumping ground for every leftover I can scrounge out of the fridge.  Quiche also lends itself well to use of seasonal ingredients, which can be handy for those of you with CSA shares.  You can add asparagus in spring, corn in summer, squash in fall, and swiss chard in winter.  Here's a run-down on some other common quiche ingredients:

Cheese: although the traditional Quiche Lorraine does not contain cheese, many people (myself included) feel that a cheese without quiche is useless.  For a basic quiche, add eight ounces of shredded cheddar.  You can also use swiss or gruyere cheese, or mix and match - cheddar plus a bit of feta; swiss plus a bit of bleu cheese.  Go nuts!

Onions: I consider onions to be another mandatory ingredient.  Chop fine and brown in a pan first.  If you're feeling adventurous, use leeks, green onions, or scallions instead.  

Garlic: Okay, garlic is also mandatory in my quiches, but I'm a garlic lover.  I usually add 3-4 cloves (minced) but the flavor can be a little overwhelming.  One clove might be better, for mixed company.

Spinach: Frozen shredded spinach is probably the norm, but it's kind of gloppy, don't you think?  Try buying fresh spinach (regular or baby), chopping it into shreds, and sautéing it until soft.  The flavor and nutritional difference will be remarkable.

Mushrooms: Avoid canned mushrooms.  Those are just gross.  I like to split my mushrooms into two groups: I take 80% of the mushrooms and dice them into little chunks.  Then I slice the remaining 20% into nice, "mushroom-shaped" slices.  You can add these "raw," but I prefer to brown them in the skillet first.

Peas and Beans: Baby peas are about the only vegetable I buy frozen anymore. (Is it even possible to buy fresh baby peas, pre-shucked?)  I thaw the peas and add them without cooking.  Fresh beans are another good addition, but you will want to dice them up and soften them on the stovetop first.