I begin my onion soup by checking for "flat ribs" at the market. I buy them when they are cheapest. I go home and roast them with a rub I make. I gnaw on the ribs until I've had my fill and throw the bones in the freezer. When I run out of room in my freezer, it's stock making time. I usually plan this for some Saturday when I have indoor chores.
Get out the biggest, heaviest pot you have and throw the bones in first, no liquid, not even thawed. Heat this up (med. med-hi) until you can smell it and little bits are starting to stick to the pan. Throw in celery, carrots, onions and garlic, these can be in good size chunks. (Don't forget a good pinch or two of kosher salt and a few cranks of cracked pepper.)You may need a bit of olive oil at this point, just to keep things from sticking too much. (Not too much) Rattle these things around until it seems as though each item has gotten a little browned.
Here comes the water. I use enough to top my ingredients by at least as high as the ingredients are deep. So, by that rule of thumb, if your ingredients are about 3 inches deep in your pot, add water to, at least, 6 inches.
I hate to let this boil as I don't like to skim for the "scum". So, I keep it just below boil for awhile (say 20 mins.), then cover and simmer for awhile ( as long as it takes to do your other chores, at least two hours.) Remove the cover, let it reduce for awhile. This timing is now dependent on your taste. I keep a ladle and a mug by the pot to taste until it's the strength I want. I set the timer for 45 mins to an hour and check on it through out the afternoon.
This process takes you up to supper time, (if you start late morning.) Set aside this pot to let it get cool enough to handle. This will be an hour or so.
Line a colander with cheesecloth and pour your first pot through this into either another pot (smaller this time) or a bowl. Now, the broth needs to cool down enough to put in the fridge. I usually do this right before bedtime.
Next day, take the fat layer off with a spatula. This broth should be good and jellied. It is ready to use or freeze or split and do both. You will reduce it more when you actually make a soup with it.
Considering that I love my roasted flat ribs (cheap cheap) and will have quite a few meals of them first, this stock is really a bonus $-wise. ( I even dry out the boiled bones and save them as treats for my dog. Talk about cheap!)
So, that's the best stock I know. Always the key to a good onion soup.
I use large sweet spanish onions, sliced as paper thin as possible, enough butter to keep the onions moist until golden and my secret (no longer) ingredient is a good splash or two of vermouth on the onions a minute or so before I add the fabulous home-made beef stock. This should simmer until it tastes the way you want it. (Correct for salt and pepper. I never add herbs, but if you do, do it at the end to avoid them turning bitter.
I love to try artisanal breads, like rosemary-garlic, or olive or any of those hearty breads that even supermarkets are making, for the croutons. I toast (or stale them if I think of it early enough) slices and cut them onto bowl-fitting shapes.
For cheese, it's Jarlsberg or nothing for me. I love the stringy, chewy, strong flavor.
Float the crouton on the soup, top with piled shavings of cheese, broil till it's deep golden brown, (even almost scorched, if you dare.)
So, I think I beat everybody in the time factor, as this can take WEEKS starting with the rib suppers.