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How thick should I slice onions for French onion soup?

Onion soup is a delicious and comforting soup that is popular in France and around the world. French onion soup in particular is known for its deep, rich flavor that comes from slowly caramelizing onions in butter or oil. One of the keys to achieving that iconic flavor is properly preparing the onions that form the base of the soup.

Why Onion Slice Thickness Matters

The thickness at which you slice the onions for French onion soup directly impacts the final texture and flavor of the dish. Thinner slices will caramelize faster and melt into the broth, while thicker slices will maintain more structure and onion bite. There are benefits to both thin and thick slices, so the ideal thickness comes down to personal preference.

Here are some factors to consider when deciding how thick to slice onions for French onion soup:

  • Thinner sliced onions will cook faster – This allows them to caramelize and melt into the broth quickly for a smooth texture.
  • Thicker sliced onions maintain more bite – This gives you some nice onion chunks in each bite of the finished soup.
  • Uniform slicing leads to even cooking – A consistent thickness allows the onions to cook at the same rate.
  • Onion variety impacts ideal thickness – Stronger, more pungent onions can handle thicker slicing while milder ones are best thinly sliced.

Recommended Onion Slice Thickness

When preparing classic French onion soup, a medium onion slice thickness between 1/4-inch to 1/2-inch is recommended. This strikes the right balance between caramelized, melty onions and maintaining some texture.

Here is a more detailed breakdown of common onion slice thicknesses for French onion soup and their results:

Onion Slice Thickness Texture and Flavor Results
1/8 inch or 3mm Very thin slices that will cook quickly into a smooth, mild onion flavor.
1/4 inch or 6mm Balances melting into the broth with maintaining some onion bite.
1/2 inch or 12mm Thicker slice gives the most pronounced onion flavor and chunks in the soup.

As you can see, the 1/4 to 1/2 inch range hits the sweet spot for great French onion soup texture. You still get some dissolved onion to thicken the broth, while retaining enough substantial slices for onion flavor in each bite.

Variations Based on Onion Type

The variety of onion used can also impact the ideal thickness for French onion soup. Here are some guidelines based on onion type:

  • Yellow Onions – The classic choice, slice yellow onions anywhere from 1/4 to 1/2 inch thick.
  • White Onions – Sweeter and more delicate, keep white onion slices on the thinner side at 1/8 to 1/4 inch.
  • Red Onions – Their sharp flavor stands out, so they can be sliced thicker at around 1/2 inch pieces.
  • Shallots – More nuanced flavor, slice shallots thin at 1/8 to 1/4 inch for even cooking.

Adjust your onion slice thickness based on the varietal to balance sweetness and pronounced onion flavor. Stronger onions can handle thicker slices while milder ones are best thinly sliced.

Cutting Onions for Optimal Caramelization

In addition to thickness, the way you slice onions can impact how well they caramelize for French onion soup. Here are some tips for cutting onions to maximize browning and flavor:

  • Slice pole to pole – This exposes more surface area to brown during cooking.
  • Leave root end intact – Helps onion slices hold together instead of separating into rings.
  • Use a sharp knife – Clean cuts prevent tearing that can lead to uneven cooking.
  • Cut uniformly – Consistent sizing leads to even caramelization.

Taking care when slicing onions can lead to better maillard browning and incorporates into the broth smoothly. This results in a rich, full-bodied flavor.

Choosing the Right Cooking Vessel

The type of pan you use to caramelize and cook the onions also impacts the final onion slice thickness you should aim for. Here are some guidelines based on vessel:

Pan/Pot Type Recommended Onion Thickness
Skillet or Sauté Pan Thinner – 1/8 to 1/4 inch
Dutch Oven Medium – 1/4 to 1/2 inch
Slow Cooker Thicker – 1/2 inch plus

The tighter heating of a skillet lends itself to thinner onions that will melt down. While a roomy dutch oven or slow cooker can handle thicker sliced onions over the long cooking time.

Cooking Onions Thoroughly

Regardless of onion slice thickness, be sure to cook the onions thoroughly when making French onion soup. Well caramelized onions are essential for developing the deep, rich flavor. Undercooked onions will taste raw and harsh.

Here are some tips for fully cooking onions:

  • Cook low and slow – Slow caramelization over medium-low heat delivers the best flavor.
  • Stir frequently – Prevent burning by stirring onions every few minutes during cooking.
  • Cook until deep brown – The onions should be a deep mahogany brown, not blond or yellow.
  • Add a splash of stock or wine – Helps accelerate the caramelization process.

Patience is key, so be prepared to cook the onions for 45-60 minutes to achieve the right flavor profile. The time investment is well worth the end result!


Finding the ideal onion slice thickness for French onion soup comes down to personal preference and the specific recipe. For a classic soup, slices between 1/4-1/2 inch deliver the right balance of melted onions and hearty chunks. Thinner slices result in a more integrated, velvety texture, while thicker slices maintain pronounced onion bites. Adjust slicing based on onion variety, cooking vessel, and desired final texture. No matter what thickness you choose, be sure to caramelize the onions fully to develop the rich, sweet depth French onion soup is known for.