Saute 1 thinly sliced red onion in 1/4 cup olive oil in a large skillet over medium-high heat, 5 minutes. Stir in 2 tablespoons red wine vinegar. Add 1 roughly chopped head escarole and 3/4 teaspoon kosher salt. Cover and cook until wilted, 5 minutes. Uncover and cook until any excess liquid evaporates.
Photograph by Antonis Achilleos