How To Make Vegetable Stock

What You Need

Ingredients
1 to 2 onions
2 to 3 carrots
3 to 4 celery stalks
4 to 5 sprigs fresh thyme
1 bay leaf
1 small bunch parsley
1 teaspoon whole peppercorns
Optional Extras: leeks (especially the green parts), fennel, tomatoes, mushrooms, mushroom stems, parsnips
Equipment
Sharp knife
Stock pot
Strainer
Cheesecloth or coffee filters (for straining)
Storage containers

Instructions

1. Gather Some Vegetables and Herbs: Onions, carrots, and celery give stock a great base flavor, and you can round these out with any of the other vegetables listed above. You can also make stock using any amount of vegetables that you happen to have on-hand, but it's good to have a roughly equal portion of each so the resulting stock will have a balanced flavor.
It's nice to add a few herbs to the stock, but we tend to keep them fairly light. Parsley does really well, especially the stems leftover from picking off the tops. Bay leaf adds a pungent, earthy flavor and thyme gives a nice woody note.
2. Roughly Chop All The Vegetables: Wash any visible dirt off the vegetables and give them a rough chop. You don't even need to peel them first unless you really want to. (Some people even advocate leaving on the onion skins!) Throw all the vegetables in a pot big enough to hold them plus a few extra inches of water.
3. Cover with Water and Simmer: Cover the vegetables with enough water that you can easily stir them in the pot. Less water means that your stock will be more concentrated; more water makes a lighter-flavored stock. Set the pot over medium-high heat and bring it to just under a boil. Once you start to see some bubbling around the edges of the pot and a few wisps of steam on the surface, turn the heat down to medium-low.
4. Cook for One Hour or So: This isn't an exact science, but one hour is generally enough time to infuse the water with vegetable goodness. If you need to take it off the heat a little early or don't get to it until a little later, it will be fine. Give it a stir every now and again to circulate the vegetables.
5. Strain and Store Take the pot off the stove and remove all the vegetables with a slotted spoon. Set your strainer over a big bowl and line it with cheese cloth or coffee filters. Pour the stock through. Divide the stock into storage containers, cool completely, and then freeze.
Additional Notes:
• Roasting and Sweating - Two ways to add more flavor to your broth are to roast the vegetables beforehand or to let them sweat (start to soften and release their liquids) for a few minutes over the heat before adding the water.
• Saving Vegetables for Broth - We keep a big sealable bag in our freezer where we can throw vegetable odds and ends: vegetables that have wilted beyond saving, the green parts from leeks, trimmings from carrots, and so on. Once this bag gets full, we use the contents to make broth. 

Read more at :
http://www.thekitchn.com/how-to-make-vegetable-stock-cooking-lessons-from-the-kitchn-136725