Vegetable Stock in the Slow Cooker

There is a chill in the air — okay, it's in the 70s — and I'm thinking ahead to a long winter of soups and stews.

The secret to a good vegetarian soup is an excellent stock. While there are many vegetable stocks on the shelves of the grocery store, I prefer to make my own so I can control the ingredients (no bht or msg or "natural flavors"). I make a good strong stock and freeze it in 1 1/2 cup containers. They are so flavorful that one container added to 4 1/2 cups of water makes a really good base for soup.

The vegetables used to make stock impart all their goodness to the liquid, and are pretty much tasteless mush at the end. I happily give them to the compost heap - they are full of fiber and minerals that feed the compost microbes. They turn it into black gold that I then spread on the garden.

The vegetable stock I stash in the freezer is a form of gold too. It will enrich every soup we eat this winter.

The key to a good stock is roasting the vegetables before simmering them. Think of the difference between golden sautéd onions and boiled onions. A few dried mushrooms contribute an almost beef-like richness. Vary the other vegetables, but if you find an organic parsnip make sure to throw it in. (I confess that I have hoarded last year's parsnips in my fridge all summer, pulling them out to make stock as needed. They get a little weird, but after peeling they're still good.)

Also, while a stock is a good place for diverse vegetables, don't add brassicas — they change the flavor and can get metallic-tasting when frozen. Keep to the base of onions, potatoes and carrots and your stock will enhance whatever soup (or risotto) you're making.

(When I buy fresh shiitake mushrooms, I wash and remove the stems, then dry them on a rack in the kitchen. When I add them to the stock, I feel like I'm getting free flavor!)

Here's the recipe for the stock I made this week.

Vegetable Stock
1 large onion
2 large carrots
1 large potato
1 large parsnip
1 stalk celery
3 garlic cloves
1 tbsp olive oil
salt and pepper
4 dried shiitake or porcini mushrooms (or an equal amount of dried stalks)
2 bay leaves
1/2 tsp black peppercorns
1 tbsp tamari

Preheat oven to 450°F. 

Remove the papery peel from the onion and slice it thickly. Scrub the carrots and potatoes and cut in 1" chunks. Peel the parsnip and cut in 1" chunks. Chop the celery in 1" lengths. Place the garlic cloves, unpeeled, on your cutting board and crush with the side of your big knife. 

Place the vegetables in a lightly oiled baking pan. Drizzle with the olive oil and sprinkle with salt and pepper. Roast until lightly browned, turning once, about 30 minutes total.

Meanwhile, soak the dried mushrooms in 1 cup water until softened. Remove them from the water and check for dirt or grit. Pour the soaking water through a coffee filter, straining out any grit. Reserve mushrooms and soaking water.

Put the roasted vegetables in a 4- to 6-quart slow cooker. Add mushrooms and soaking water, bay leaves, peppercorns, tamari, 1 tsp salt, and 7 cups water. Cover and cook on low for 8 hours.

Let cool slightly, then strain through a fine mesh sieve into a large bowl. Press the vegetables against the side of the sieve with a wooden spoon to get the most liquid possible, although this will make the stock cloudy, so don't do it if you need a clear broth. Discard the vegetables.

Either use the stock right away, refrigerate it for a few days, or freeze for longer storage.

Makes about 8 cups.