Classic Onion Soup

This classic onion soup celebrates the humble onion in all its glory. Slowly caramelized until deeply golden and sweet, swimming in rich beef broth, it's a testament to how simple ingredients can create something extraordinary. This is the soup upon which French onion soup is built—and it's magnificent on its own.

The Art of Caramelizing Onions

Caramelization is a chemical process where heat causes the sugars in onions to break down and recombine into hundreds of new flavor compounds. The result is a transformation from sharp, pungent raw onion to something sweet, complex, and deeply savory.

True caramelization cannot be rushed. At least 30 minutes is necessary, often 40 or more. During this time, the onions progress through distinct stages:

• Minutes 0-10: Onions release moisture and begin to soften, becoming translucent
• Minutes 10-20: Moisture evaporates, onions start to shrink and turn pale gold
• Minutes 20-30: Color deepens to golden, sweetness develops, fond builds on pot
• Minutes 30-40+: Deep amber color, intense sweetness, complex flavor

The fond—those browned bits stuck to the bottom of the pot—is concentrated flavor. Don't scrape it up during cooking; instead, let it accumulate and deglaze it with wine at the end for maximum flavor extraction.

The Science Behind the Sweetness

Raw onions contain about 5% sugar by weight, but it's masked by sulfur compounds that create the familiar sharp, eye-watering effect. Heat breaks down these sulfur compounds while concentrating the sugars through moisture evaporation. The Maillard reaction—the same process that browns a steak—creates additional flavor compounds.

Adding a small amount of sugar isn't cheating—it simply jump-starts the caramelization process and helps develop more color. The finished soup shouldn't taste sweet; the sugar integrates into the overall savory profile.

Choosing the Right Onions

Yellow onions are the classic choice and what we recommend. They have an ideal balance of sugar and sulfur compounds that creates the best flavor when caramelized.

Sweet onions (Vidalia, Walla Walla, Maui) can be used but produce a sweeter, less complex soup. They also have higher water content and take longer to caramelize.

Red onions turn an unappetizing grayish-purple when cooked in liquid, so avoid them for this recipe.

White onions are sharper and don't develop as much sweetness. They can be used but need extra cooking time.

Why White Wine?

The wine serves multiple purposes. Its acidity helps deglaze the pot, dissolving the flavorful fond into the soup. The alcohol extracts additional flavor compounds from the onions. And the wine's subtle fruity notes add another layer of complexity to the finished soup.

Choose a dry white wine you'd actually drink—Sauvignon Blanc, Pinot Grigio, or dry Chardonnay work well. Avoid anything sweet or heavily oaked. No wine? Substitute with dry sherry, or simply use additional broth with a splash of white wine vinegar.

Tips for Perfect Caramelization

• Use a wide, heavy pot: More surface area means faster moisture evaporation
• Keep heat at medium or medium-low: High heat burns instead of caramelizes
• Don't stir too frequently: Let onions sit to develop color, then stir to prevent burning
• Add water if needed: If fond starts to burn, add a splash of water and scrape
• Be patient: There are no shortcuts to proper caramelization
• Slice uniformly: Even slices cook evenly

Making It French Onion

To transform this into classic French onion soup (soupe à l'oignon gratinée):

1. Ladle the finished soup into oven-safe bowls or crocks
2. Float a slice of toasted baguette on top (dry it in the oven first so it doesn't get soggy)
3. Cover generously with grated Gruyère cheese (about 1/2 cup per bowl)
4. Place under a hot broiler for 2-3 minutes until cheese is bubbly and golden
5. Serve immediately—the bowls will be extremely hot

Storage and Make-Ahead

This soup is actually better the next day. The flavors deepen and meld overnight in the refrigerator. It keeps well for up to 5 days refrigerated and freezes beautifully for up to 3 months. If converting to French onion soup, add the bread and cheese just before serving.