How to Prevent Roasted Red Onions from Discoloring

Red onions, a common and delicious part of many sheet pan dinners, often lose their bright color during oven roasting. The dull, gray veggies may taste great, but do not look too appetizing on your plate. Luckily, there is an easy way to fix this predicament.

The solution is to increase the acidity of the onion’s cooking environment. An easy way to do this is by adding a splash of red wine vinegar before roasting. In our kitchen test, the onions with the vinegar held onto their beautiful natural red color after a stint in the oven. The untreated onions, while still tasty, do not look so appealing. Other acidic ingredients will also work, such as the fresh lemon juice in our sheet pan rustic potatoes recipe. Here’s how you can make a batch of roasted red onions without losing all of their vibrant color.

Pro Tip: Reusable Silpat-type silicone sheet pan mats help keep cleanup easy and are also great for baking applications. Here are the mats I use from Amazon.

Ingredients for Sheet Pan Roasted Red Onions

1 large red onion, cut into thin wedges
2 T. extra virgin olive oil
2 T. red wine vinegar
Kosher salt and black pepper, to taste

How to Make Oven Roasted Red Onions

1. Place the top oven rack in the center position and preheat oven to 400°F. Line a large, rimmed baking sheet with parchment paper or a silicone baking mat. Set aside.
2. Add the sliced onions to a medium mixing bowl and top with olive oil and red wine vinegar. Season with salt and black pepper, to taste. Gently toss to coat and transfer to the baking sheet. Spread into a thin, even layer without overcrowding.
3. Place baking sheet in the preheated oven for 30 minutes, turning once after 15 minutes. Remove from oven and cool slightly before handling.