Bring a large pot of salted water to a boil. Add the escarole and cook until bright green, about 1 minute. Drain and transfer to a bowl of ice water; drain and squeeze dry.
Heat the olive oil in a large skillet over medium heat. Add the onions and cook until soft, about 6 minutes. Add the escarole and tomatoes, reduce the heat to low and cook until tender, about 6 minutes. Add the feta and cook until partially melted, about 5 more minutes. Season with salt and pepper.
Photograph by Jim Franco
Courtesy of Michael Symon for Food Network Magazine