A slow cooker makes delicious onion soup, or you can stop at caramelized onions and use them in other recipes.
A slow cooker makes delicious onion soup, or you can stop at caramelized onions and use them in other recipes.Matthew Mead/Associated Press

And it’s not just for taste. The onion also happens to be a terrifically healthy ingredient. If nothing else, it’s a delicious way to add bulk to a dish without adding many calories or any fat. Thing is, Americans typically use onions in such small quantities that they serve more to highlight the other items in a dish, rather than strut their own stuff.

The exception? A warm and satisfying bowl of onion soup.

The genesis of onion soup is unclear, but some attribute the version we know today to a French king. Even in modern France, onion soup is slurped up by late-night revelers taking a break in the party to revitalize their bodies with warm oniony broth before continuing the fete into the morning hours.

While I tend to sip my onion soup at a more reasonable hour, I find it equally comforting. There is nothing as soothing as the luscious sweetness of a cooked onion, particularly one from which a caramel color and flavor have been coaxed unhurried over hours. Add a rich broth and a bit of cheese, and you have a delicious luxury.

The secret is in not rushing the caramelizing of the onions. And that’s a fine way to put your slow cooker to work.

It conveniently caramelizes the onions overnight with no effort from you. And my recipe for slow cooker onion soup walks you through the whole (very easy) thing.

And if you want, you can stop right there – with caramelized onions! Put them in a small jar in the refrigerator and use as a spread or topping for just about anything. Or continue on and make the whole soup, which is guaranteed to nourish both body and soul.

Slow Cooker Onion Soup

Start to finish: 10 hours (plus 20 minutes active)
Servings: 6
For the caramelized onions:
5 cups sliced yellow onions, (about 4 large onions)
2 shallots, peeled and sliced
1 clove garlic, minced
1 tablespoon olive oil
Salt and ground black pepper
For the soup:
1 teaspoon all-purpose flour
¼ cup dry sherry
1 teaspoon dry thyme
Small sprig fresh rosemary
1 teaspoon Worcestershire sauce
2 cups low-sodium chicken broth
2 cups low-sodium beef broth
1 cup water
1 tablespoon lemon juice
6 thin slices baguette, toasted until golden and quite dry
½ cup grated Gruyere cheese
Method:
Coat a 4-quart or larger slow cooker with cooking spray. Add the onions, shallots, garlic and oil. Toss well, then season with a bit of salt and pepper. Cover and cook on low until the onions are richly colored and sweet, 8 to 10 hours. (The onions at this point can be removed and used for other recipes if not making the soup.)
Once the onions are ready, sprinkle them with the flour and stir until well-combined. Add the sherry, thyme, rosemary, Worcestershire sauce, both broths and the water, then cover and cook for another 4 hours on low or 2 hours on high. Once soup is done, stir in the lemon juice and adjust the seasoning.
When ready to eat, set the broiler on high. Ladle the soup into broiler-safe serving crocks. Top each crock with a thin slice of the toasted baguette and a heaping tablespoon of Gruyere. Place the filled crocks on a rimmed baking sheet and broil just until cheese is golden, 1 to 2 minutes. Do not leave the crocks unattended as they will burn quickly.
Nutrition information per serving: 240 calories; 60 calories from fat (25 percent of total calories); 6 g fat (2.5 g saturated; 0 g trans fats); 15 mg cholesterol; 34 g carbohydrate; 3 g fiber; 8 g sugar; 10 g protein; 570 mg sodium.