3 large sweet onions, such as Vidalia, cut into 1/8-inch slices
1/2 cup small-diced shallots (about 2 large)
2 garlic cloves, minced
3 cups sour cream
2 cups mayonnaise, preferably Hellmann’s
1 tablespoon celery salt
1 tablespoon Worcestershire
2 teaspoons fresh lemon juice
1/4 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
1 tablespoon minced chives
Depending on the size of the skillet, you may need to sauté in two batches. In a large skillet, melt the butter over medium-high heat until it bubbles. Add the onions and shallots and sauté, stirring occasionally, until golden brown, 20 to 25 minutes. Add the garlic in the last minute of cooking and sauté just until fragrant. Set the pan aside to cool for about 15 minutes, then transfer the onion mixture to a cutting board.
Meanwhile, in a large bowl, combine the sour cream, mayonnaise, celery salt, Worcestershire, lemon juice, kosher salt, and pepper. Whisk until well mixed.
Chop the cooled onion mixture and add it to the bowl. Fold into the mixture until well incorporated. Cover the bowl and refrigerate for at least 1 hour or up to overnight. Stir again and sprinkle with chives right before serving.
Store in an airtight container in the refrigerator for up to 2 days.