Shallots have a distinctive taste that might best be described as a garlic-onion hybrid, although this bulb isn't a genetic mix of the two. Shallots have an onion-like flavor with a bite that approximates garlic's, but with a sweeter and milder taste. Shallots readily take onion's place as an aromatic in just about any preparation. The results are a more tempered taste and -- because shallots have a finer texture -- less prominent chunks in the finished product, a particularly appealing quality in delicate or subtle dishes.
Use three shallots per small onion or 1/3 cup of chopped onion. Five to six shallots replace a medium onion, while you'll need seven or eight to stand in for a large onion. Cooking, unlike baking, doesn't require precise measurements, so you may tailor aromatic quantities to taste.
Use a bit more shallot than you would onion for a distinct flavor. Because of its milder taste, the flavor added by shallot is more subtle than the flavor added by an equal amount of onion.
Withhold most or all of the garlic in a recipe when substituting shallot for onion. Shallot offers a similar bite, so using it and garlic together easily creates too potent a garlicky flavor in a dish.