In a large bowl combine the endive, scallions, radishes, and favas. Whisk ¹⁄³ cup oil into the vinegar mixture, pour it over the vegetables, and toss to coat. Taste and adjust the seasoning with salt, as needed. If you have small, tender basil leaves, add them to the bowl whole; otherwise, roughly chop the leaves, add them to the bowl, and toss to coat.