Before shallots and garlic became the standard aromatics for dressings, vinaigrettes and dips, onion juice was often used. Onion juice is easy to make: you just grate an onion on a fine grater -- a Microplane works well -- gather the pulp in your hand and squeeze out the juice. What's great about having onion in liquid form is that you can control the amount of onion flavor you add to a recipe (also, less risk of dragon breath). Try making it next time you're assembling a salad -- I think onion juice works particularly well in buttermilk dressings, like this one:
1 teaspoon Dijon mustard
1 1/ 2 tablespoons red wine vinegar
1/ 4 cup olive oil
1/ 4 cup buttermilk
Freshly-ground black pepper
1/ 2 to 1 teaspoon onion juice
In a small bowl, whisk together the mustard, vinegar and salt. Gradually whisk in the olive oil until emulsified, then whisk in the buttermilk. Season to taste with pepper and onion juice -- and more salt, if needed.
Makes about 2/ 3 cup.