Heat oven to 350° F. On a rimmed baking sheet, toss the bread with 3 tablespoons of the oil, ½ teaspoon salt, and ¼ teaspoon pepper. Bake, tossing once, until golden, 20 to 25 minutes. Let cool.
In a large bowl, whisk together the vinegar, mustard, the remaining 5 tablespoons of oil, ¾ teaspoon salt, and ½ teaspoon pepper. Add the escarole, radicchio, fennel, and croutons and toss to combine.