Using a sharp knife, cut a thin slice away from the root and stem of the onion, but leaving the root intact.
Place the onion stem-side down on the cutting board and slice in half from the root end to the stem end. Gently peel away the outer layer and using the tip of the knife, remove any dark spots.
To dice the onion:
Lay the onion cut-side-down onto the cutting board. Using a sharp knife, make two to three horizontal cuts (parallel to the cutting board) into the onion, cutting to just before the root end. The root end will hold the onion together.
Next make a series of evenly spaced parallel, lengthwise cuts with the tip of the knife.
Lastly, holding the onion in your guiding hand (non-dominant), complete the dice by cutting even crosswise cuts working from the stem end to the root end of the onion. Be sure to cut through all layers of the onion.
To julienne the onion:
Lay the onion cut-side-down onto the cutting board. Remove the root end. With the root end or the stem end facing you, make evenly cut matchstick-sized cuts through the onion, moving in a radial fashion towards the center of the onion, changing the angle with each cut.
To slice the onion:
Lay the onion cut-side-down onto the cutting board. Using a sharp knife, make a series of cuts, slicing them to the desired thickness.
To slice onion rings:
Do not slice the onion in half as directed in step 2. Holding the onion firmly with your guiding hand, slice into the onion starting at the stem end into rings of the desired size, stopping just before the root end. Discard the root end and break up the onion slices into rings.
Types of onions: These tips for how to cut onions will work on any of the varieties below.
Yellow onions: Versatile and widely available, yellow onions have layers of white flesh and a tough, brown-yellow skin. They’re spicy and astringent, but they sweeten up beautifully when cooked.
Sweet onions: Also known as Walla Walla or Vidalia, sweet onions are often battered for onion rings and fried onion recipes. They’re higher in sugar than yellow onions and are mildly sweet in flavor.
White onions: White onions are often large with white-fleshed and thin paper-like skin. Mostly used in Mexican cuisine, they are crunchy and on the sharper side.
Red onions: Light purple-colored red onions are ideal to eat raw in salads or work well pickled or grilled.
Shallots: These small, clustered onions have pinkish-brown skin and white or light pink flesh. Milder and more delicate in flavor than most other onions, shallots are great to use when you’re planning to eat them raw. They offer a hint of garlic flavor, too.
Uniform size: While there are many options for how to slice onions, no matter which way you choose, aim for uniform pieces so they cook evenly (if applicable) or look orderly.
Buying: Choose onions that are firm to the touch and free from soft spots, blemishes, and brown spots.
Storing: Whole onions and shallots last for about 2 to 3 months when stored in a cool, dry, dark, and well-ventilated place such as a basement or the bottom of your pantry. Refrigerate peeled whole onions and shallots in an airtight container for up 10 to 14 days. Cut onions should be refrigerated and used within 7 to 10 days.