In a large pot over medium heat, heat the oil. Add the onion, garlic and sage. Cook for 5 minutes or until onions are translucent. Add the vegetable stock, beans and tomatoes to the pot. Simmer for 20 to 30 minutes.
Transfer two-thirds of the soup to a blender or food processor and process until pureed. Stir puree back into the pot. Add the escarole, salt and pepper. Cook until escarole is wilted, about 2 minutes.