Make the couscous: In a small saucepan, heat the oil over medium-high heat. Add the couscous and toast until lightly golden, about 4 minutes. Add 3/4 cup water and 1/4 teaspoon salt and bring to a boil. Lower the heat so the water simmers; cover the pan and cook until the liquid is absorbed, about 10 minutes. Uncover the pan and set aside to cool.
Make the vinaigrette: In a small bowl, whisk together the vinegar, honey, 1 1/2 teaspoons salt, 1/4 teaspoon pepper and the oil.
Make the salad: In a large bowl, toss together the apple, fennel, grapes, walnuts and couscous. Drizzle the vinaigrette over the salad, tossing until coated.
Put 1 radicchio leaf on each plate. Spoon the salad into each leaf, allowing some to spill over.