Sweet Onion Sauce

This sweet onion sauce comes together in under 15 minutes with only 6 ingredients! It’s a simple way to add a rich, tangy-sweet flavor to sandwiches, subs, salads, and more. Skip the store-bought options and try this homemade Subway copycat instead!

With a few pantry staples, you can make a sweet onion sauce that's even better than Subway's! The best part? It's easy, budget-friendly, and preservative-free. If you love all things sauce, try my Southwest salad dressing!

Why I Love This Recipe

If you’ve ever found yourself wishing you could bottle up the Subway sweet onion sauce, you’re in luck—this homemade version tastes just like the original but without any of the mystery ingredients!

The onions create a naturally sweet base, balanced with a touch of vinegar and a kick of pepper. It turns out silky smooth, slightly thickened, and packed with that irresistible bold, savory flavor.

It’s perfect for drizzling over sandwiches, salads, roasted veggies, or even as a dip. Whether you're meal-prepping or want to elevate your usual meals, this sauce is an absolute keeper!

Ingredient Notes

Here's a breakdown of the ingredients you'll need to make this sweet onion sauce recipe.

• Onion: You can use white or yellow onions, although I prefer white onions for their crisp, clean flavor. (If you can get your hands on some, I also recommend sweet onion varieties like Vidalia and Walla Walla.)
• Sugar: Balances the vinegar and enhances the onions' natural sweetness. Use cane sugar or vegan granulated sugar!
• White Wine Vinegar: I prefer the slightly sweeter, milder, and more delicate taste of white wine vinegar over regular white vinegar. It results in a well-rounded sauce rather than one overpowered with acidity.
• Poppyseeds: Just like Subway’s sweet onion sauce, don’t forget to include poppyseeds for a light crunch and a slightly nutty flavor.
• Potato Starch: Acts as a thickening agent, helping the sauce become smooth and velvety.

Substitutions

• Onion: You can try an equal amount of shallots for a milder onion flavor.
• Sugar: Use maple syrup or coconut sugar for a refined sugar-free option. Start with slightly less, then taste and adjust the sweetness gradually.
• White Wine Vinegar: Apple cider vinegar or rice vinegar will work in a pinch!
• Poppyseeds: If you don't have poppyseeds, use an equivalent amount of sesame seeds or omit them.
• Potato Starch: → Cornstarch will also work here (use a 1:1 ratio).

How to Make

My favorite part about this sweet onion sauce is how easy it is to whip up! Just follow my tips and tricks for the best results.

Preparation

In a small bowl, whisk together the potato starch and water to form a slurry. Roughly chop the onion into small pieces to make blending easier.

Cooking

Step 1
Add the onion and water to a blender and blend until completely smooth (like onion juice).

Step 2
Pour the blended onion mixture through a fine-mesh sieve, using a spoon to press out as much liquid as possible and remove the pulp. Straining the onion juice is key to achieving a silky sauce (if you want a thicker, more textured sauce, you can skip this step).

Step 3
Heat the saucepan of onion juice over low-medium, then add the slurry and the remaining ingredients to thicken the sauce.

Step 4
Bring the sauce to a quick boil, immediately turn off the heat, and let it cool (your sweet onion sauce will thicken more as it continues to cool).

Expert Tip

It's essential to heat the onion juice gently, which allows its natural sweetness to develop without caramelizing or burning (this could alter the final taste of your sweet onion sauce).

Always add the slurry gradually while stirring constantly—this will thicken the sauce evenly without clumping. And avoid cooking the sauce for too long, as it can actually break down the starch, making it thinner instead of thicker. Remove it from the heat as soon as it starts to thicken.

Serving Suggestions

Once your sweet onion dressing is cooled and thickened, enjoy it on sandwiches, subs, and paninis. You can also serve it in wraps and pitas or as a dip with pizza (bonus if it's topped with some sugar-free caramelized onions!).

Of course, you can always use it as a salad dressing in recipes like my strawberry goat cheese salad or even a quinoa salad.

Cooking Tips

• Use Fresh Onions: Fresh onions have a higher water content, which results in a smooth, flavorful sauce. Avoid pre-chopped or frozen onions, as they can alter the texture and taste.
• Blend thoroughly: The onion juice should be completely smooth before straining. A high-powered blender works best here, but if yours isn't as strong, blend for an extra 30 seconds.
• Adjust the Sweetness: Depending on the onions you use, you may need to adjust the sugar. Taste the sauce before cooling and adjust the sugar to your preference—add more for a sweeter sauce or balance it with extra vinegar.

Frequently Asked Questions

Storing

If you have leftover sweet onion sauce, you’re in luck! It can be stored for quite a long time by following these storage instructions.

• Fridge: Your sweet onion dressing will stay fresh for up to a month in the fridge if kept in an airtight container (I prefer a mason jar here!).
• Freezer: It can also be frozen in small portions for up to 3 months. Use an ice cube tray to freeze individual servings, then transfer them to a freezer-safe bag or container for easy use.
• Thaw: When ready for more, thaw a cube in the fridge overnight. Give it a quick stir in the morning, especially if it’s separated.

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