Few ingredients are as broadly cherished in the world's cuisines as onions, garlic and their diverse kin. Though they all share broadly similar characteristics, the plants in this family -- collectively known as alliums -- generate a remarkably diverse set of flavors. They range from mild and sweet to eye-wateringly pungent, providing an ingredient for almost any culinary purpose. Two of the mildest and most versatile are scallions and shallots, each of which provides a pleasantly light onion note.
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A Quick Flavor Primer
The onion family's flavors are created by volatile, sulfur-based compounds, which -- ironically -- evolved as a chemical defense to keep the plants from being eaten. When a plant's cells are crushed or cut, those volatile compounds are released, reacting with each other and atmospheric oxygen to create the familiar and eye-watering onion bouquet. Scallions and shallots both contain these tear-inducing chemicals, though in general they're less pungent than other onion varieties.
Let's Get Physical
Both scallions and shallots are immediately recognizable, because they're physically distinct from other onions. Scallions are the familiar "green onions" sold in bunches at the supermarket, or chopped into green rings at the salad bar. They're ordinary onions, harvested while immature and still cylindrical, resembling pencil-thin leeks. Shallots are very different, consisting of small garliclike bulbs shrouded in a tough brown skin. Shallots typically produce clusters of two bulbs together, but despite their garliclike growth habit the bulbs themselves have unmistakably onionlike layers.
Harnessing the Scallion
Scallions are most often used raw in the Western world, as their delicate chivelike flavor and texture don't need to be mellowed by cooking. They lend a pungent and savory note to salads and egg dishes, and make a vivid and flavorful garnish when sliced into thin rings and scattered with a generous hand. In cookery, they're typically added to subtly flavored dishes such as quiches or soups; there their delicacy can be showcased. Asian cooks also utilize the scallion in soups, and in general are more likely to use it as a cooked ingredient. Scallions are often added to stir-fries, or utilized alongside ginger and garlic as a flavor base for broths and sauces.
Take Your Best Shallot
Shallots, on the other hand, are almost invariably cooked. Their flavor is milder and more complex than those of other onions, and has a definite sweetness. In classical French cuisine, finely diced shallots are often infused in wine or butter to provide the flavor base for sauces. Butter has a strong affinity with shallots, enriching and enhancing their flavor as they cook gently, so the two are often paired in Western cuisine. Asian cooks also value the shallot's sweet flavor, and it's especially prominent in the foods of Southeast Asian nations, such as Vietnam and Thailand.