To achieve an amazing gourmet flavor, use only the best ingredients. Purists will tell you that anything other than homemade beef stock is garbage. Homemade stock adds that (french phrase meaning "I don't know") to your soup.
If you have the time, you definitely ought to make your own stock (this is Instructables, after all) but I'm using it from a can/box combo. That Said, here is what you need. (I apologize I do not have a mise en place photo, but I realized this would make a good Instructable as I was cutting the onions. And if you don't know what an onion is, you have far more to learn than this Instructable will provide.)
Onions! Six or so largish ones. About the size of a grapefrut. I use a mix of yellow and red because I like yellow and red. These are the onions that have some sweetness to them, which is what you want in the soup. The color won't matter in the very end, so its all a taste issue.
Butter: If you're bothering to chef it and you're using canned broth, use the real butter. Oil works well, too, but you need one with a high smoke point and that can sometimes equal less flavor. Of course, a good oil means your dish can be more vegetarian friendly. Which is a nice thing. You only need three tablespoons, but you can add more if you want to.
Salt! I use Kosher because I like for people to think I am fancy. You'll only need about a tablespoon so if you are using "normal" salt, kick this back to about a teaspoon.
Beef Stock - try to make sure you have stock, not broth. I find stock tastes way better. Any commenter can respond with the science if they want - I would be interested. You'll need about four cups.
Any Stock: Can work. Tradition calls for the beef kind, but there's nothing wrong with chicken if you've got it or even vegetable - this dish can easily be vegan if you want it to.
Wine: White, red, whatever! If you're like me (and if you are, we should totally hang out - I need non-baby/wife companionship) then you have a bottle of wine your wife drinks a glass out of and forgets about because she's on call and can't drink. My poor wife :( But it leaves me with lots of wine for cooking, and it tastes great! You only need a cup, so you can always open a fresh bottle if you're that type. Just remember you're working with blazing hot metal as you drink.
Herbage: Parsely, sage, rosemary, thyme? Definitely some thyme. It is great stuff. I use the dried, but fresh would be terrific as well. I find dried is really not too different in a soup preparation so long as it's not too old. You'll want a bay leaf, fo-sho. That's my choice for this particular dish, but it won't suffer if you love the flavor of tarragon or other herbs.
Baguette: A nice crusty loaf. You should totally make one yourself. They can be had at a lot of grocery stores these days near the checkout, and they work well - but homemade bread is amazingly easy.
Cheese: Gruyere is a great choice. I don't have it, so I'm using shredded mozzarella. I know, right? You can make your own mozzarella, if you really want to. Toss a little parmesean on as well.
A big ol' Dutch Oven or oven safe pot with a tight fitting lid. We're baking/roasting these onions. Cool, eh?
Oven proof crocks: These can be large ramekins or cool ceramic bowls. They just need to be able to withstand broiling temperatures.
Some helpful Instructables (For you Extreme DIYers - Someday I will join you):
(Note: I don't know these folks, but I thought some readers might like to really make it all for themselves, and so found these. They will help make your slow soup even slower)