I generally make stock rather than broth. I save pretty much all of my veggies that are a tad past their prime or are just "extra" I toss them in a zipper bag in the freezer. When the bag is full and I have chicken bones, necks, organs, etc (I also keep them in a zipper bag in the freezer till I'm ready), I lay everything into a large roasting pan. I always make sure I have onion, carrot and celery, the others are optional. I also throw in a handful of parsley or whatever other herbs I have growing and of which we like the taste. I add 2 or 3 cups of water and stick the roaster in the oven about about 350 till everything is good and brown. Then, I toss it all into a pressure cooker, add more water and process it for 45 mins or thereabouts.
When processing is done, I strain the liquid out and add salt/pepper to taste. Into jars and the pressure canner. We're at about 5000 feet here, so I can at about 12 lbs...75 mins for pints and 90 for quarts. I use both sizes because if I'm making soup, I'll need at least a quart, and other times I don't need more than a cup or two.
I also use up all the solids. The bones, after being pressure cooked, are soft and don't splinter. They can be added to feed either for the dogs or chickens. I actually save them in a big bag and when I have enough I throw them in the food processor and grind them up...bone meal! All the veggies go into the chicken run...what they don't eat will compost nicely right there.
For beef stock, I follow the same procedure. I keep separate freezer bags for beef and chicken. The beef bones don't get quite soft like the chicken bones so they often end up having to be tossed

Otherwise, pretty much nothing gets thrown away.
Veg stock is also a great thing to have around...you can add it to just about anything for extra flavor. I handle it just the same way...you can use whatever veggies you have on hand and/or particularly like.
Wow, Jusme...somehow I missed your post...that's an awesome idea about making the boullion! It would be such an efficient use of space! I love the convenience of bullion, but the stuff you buy seems to be nothing but various forms of salt...ugh!