Martha adapted this dish from chef Mike Price of Market Table restaurant in New York.
Place a large skillet over medium heat. Add oil, butter, and garlic; cook until garlic is golden brown. Add fennel and onion and cook, stirring, until almost tender and caramelized, 10 to 15 minutes. Add escarole, oregano, and red-pepper flakes; season with salt. Cook until escarole is wilted, 3 to 4 minutes. Serve immediately.