Green Beans with Caramelized Onion Vinaigrette

Ingredients

• 1 tablespoon unsalted butter
• 1/2 cup plus 2 tablespoons olive oil
• 1/2 large yellow onion, finely diced
• 1 teaspoon sugar
• 1 teaspoon kosher salt
• 1/2 cup sherry vinegar
• 1/4 teaspoon black pepper
• 2 pounds green beans, trimmed
• 2 teaspoons grated orange zest

Preparation

1. In a large skillet, over medium heat, heat the butter and 2 tablespoons of the oil. Add the onion, sugar, and 1/2 teaspoon of the salt. Cook, stirring occasionally, for 25 minutes. Add the vinegar. Cook, stirring, for 1 minute. Add the remaining oil, 1/4 teaspoon of the salt, and the pepper. Transfer half the mixture to a blender and puree, then return to skillet.
   
   Meanwhile, cook the green beans with the remaining 1/4 teaspoon of salt in boiling water until tender, about 4 minutes. Transfer to a bowl of ice water. Drain and pat dry. Transfer to a serving bowl, toss with the onion mixture, and sprinkle with the zest.
   
   Make Ahead: Cook the green beans and make the vinaigrette early in the day; refrigerate them separately. Combine them just before serving.