Supermarket vegetable broth is perfect in a pinch, but pales in comparison to the homemade stuff. It takes less than 10 minutes to get the stock on the stove, and the ingredients cost less than $5 to make a gallon. This leaves very little room for excuses.
Stock is an excellent way to use up sad, old veggies. I keep a bag in my freezer as a landing pad for shriveled carrots and trimmings, onion trimmings (including the peel), the green parts of leeks, mushroom stems, peppers, and herbs. After a few weeks I just toss them into the pot, cover with water and add a few aromatics: dried herbs, salt, pepper, bay leaves, whole allspice...whatever you fancy.
To keep your stock fill quart-sized Ziplock bags and lay flat on a plate or small tray. Stack the bags on top of each other and freeze. When solid store in the freezer like books on a bookshelf. You can also pour the stock into ice cube trays for smaller quantities to use in recipes that call for a little veggie broth.
1 to 2 onions (no need to skin, just whack into about 1-inch pieces)
2 to 3 carrots, roughly chopped (no need to peel)
3 to 4 celery stalks, roughly chopped (tops, bottoms and the leaves)
4 to 5 sprigs fresh thyme (or 2 tsp dried thyme)
1 bay leaf
1 small bunch parsley
1 teaspoon whole peppercorns (or 1/2 teaspoon ground black pepper)
1 tsp sea salt (or kosher)
Optional Extras: leeks (especially the green parts), fennel, tomatoes, mushrooms, mushroom stems, parsnips
1. Roughly chop all of your vegetables into about 1/2- to 1-inch pieces. It doesn't matter what veg you use, just make sure you have approximately the same amount of each so the stock has a balanced flavor. Avoid strong-flavors like cabbage, broccoli or cauliflower.
2. Toss all the veg into a stock pot--any large pot will do--and cover with enough water so you can easily stir them around, plus a few more inches. It isn't an exact science, but the more water you have the less concentrated the stock will be.
4. Cook for about an hour and a half, stirring occasionally.
5. When the stock has a nice light brown color with all of the lovely smells of a proper broth, take the pot off the stove and remove the veg with a slotted spoon. Place your strainer over a large bowl or container of choice and, using a ladle or small bowl, pour the stock through the strainer.
6. Divide the stock into containers or resealable bags and cool completely. Freeze for up to 6 months.