The Complete Guide to Cooking Onions

Onions are the foundation of flavor in almost every savory dish you'll ever cook. French cuisine calls them part of "mirepoix." Italian cooking uses "soffritto." Cajun food relies on the "holy trinity." Different names, same principle: onions build depth, sweetness, and complexity that nothing else can replicate.

But here's what most home cooks don't understand: not all onions are the same, and how you cook them completely changes what they contribute to a dish. Raw onions are sharp and pungent. Sautéed onions are soft and sweet. Caramelized onions are deeply savory and almost candy-like. The same ingredient, three completely different flavors.

I learned this on my first day at Purple Café in Seattle. The chef handed me a case of yellow onions and said, "Slice thin. Cook slow. You'll know they're done when they're mahogany brown and smell like the best thing you've ever made." It took me 90 minutes to properly caramelize five pounds of onions, and at the time, I thought it was ridiculous. But when I tasted the finished product—deep, sweet, complex, with no sharpness or bitterness—I understood. Time transforms onions from harsh to magical.

Understanding Onion Varieties

Not all onions are interchangeable. Each variety has a different flavor profile, water content, and ideal use case.

Yellow Onions (All-Purpose)

Flavor: Balanced—sharp when raw, sweet when cooked
Water content: Moderate
Best for: Sautéing, caramelizing, soups, stews, general cooking

Why chefs use them: Yellow onions are the workhorse of the kitchen. They have enough sharpness to add bite when raw, enough sugar to caramelize beautifully, and enough structure to hold up to long cooking. At Mellow Mushroom, we went through cases of yellow onions every week for everything from pizza sauce to soup bases.

When to use them: Anytime a recipe just says "onion" without specifying, use yellow. They're the default for a reason.

White Onions

Flavor: Sharper and more pungent than yellow onions, less sweet
Water content: High
Best for: Mexican and Latin American dishes, fresh salsas, raw applications

Why chefs use them: White onions have a cleaner, crisper bite that works beautifully in raw applications like pico de gallo, ceviche, and fresh salsas. They're less sweet than yellow onions, so they don't compete with other flavors—they just add onion-forward sharpness.

Red Onions (Purple Onions)

Flavor: Mild, slightly sweet, less sharp than yellow or white
Water content: Moderate to high
Best for: Raw applications (salads, sandwiches, burgers), pickling, grilling

Why chefs use them: Red onions are beautiful—deep purple layers that add visual appeal. They're also milder than yellow or white onions, making them perfect for raw applications where you want onion flavor without overwhelming sharpness. They lose their color when cooked, so they're not ideal for cooked dishes unless aesthetics don't matter.

Shallots

Flavor: Delicate, sweet, hint of garlic
Water content: Moderate
Best for: Vinaigrettes, sauces, French cooking, anywhere you want subtle onion flavor

Why chefs use them: Shallots are smaller, milder, and more refined than regular onions. They add complexity without dominating. At Purple Café, we used shallots in vinaigrettes, pan sauces, and anywhere we needed onion flavor that wouldn't overpower delicate proteins like fish.

Cooking Methods for Onions

1. Sautéing (Quick Cooking)

Time: 5-8 minutes
Result: Soft, translucent, sweet but still with some bite
Best for: Building flavor bases for soups, sauces, stir-fries

2. Caramelizing (Low and Slow)

Time: 45-60 minutes
Result: Deep golden brown, jammy, sweet, complex
Best for: French onion soup, burger toppings, flatbreads, anywhere you want deep onion flavor

For the complete technique, see The Science of Caramelization.

3. Roasting

Time: 25-35 minutes at 400-425°F
Result: Caramelized edges, soft centers, concentrated sweetness
Best for: Side dishes, salads, grain bowls

The Bottom Line: Onions Build Flavor

After 24 years of cooking professionally, I can tell you this: Onions are the foundation of almost every great savory dish.

Choose the right variety:

• Yellow - All-purpose, caramelizing, cooking
• White - Mexican food, fresh salsas, raw applications
• Red - Salads, burgers, sandwiches, mild flavor
• Shallots - Sauces, vinaigrettes, refined dishes

Cook them properly (patience for caramelization, gentle heat for sautéing), and onions will transform your cooking.