1. Preheat the oven to 375F. Whisk together the shallot, citrus juices, honey, mustard, salt, and white pepper; let stand for 2 minutes. Add the extra- virgin olive oil in a slow, steady stream, whisking until blended. Set aside the dressing.

2. Toss together the arugula, Bibb lettuce, apple, fennel bulb, and walnuts in a large bowl. Set aside the salad.

3. Place the baguette slices in a single layer on a parchment paper-lined baking sheet, and lightly brush the tops with the olive oil. Bake until lightly toasted, about 2 minutes. Remove from the oven.

4. Stir together the cheeses, lemon zest, and black pepper in a small bowl; stir in 1 tablespoon of the reserved chopped fennel fronds. Spoon about 1 tablespoon of the cheese mixture onto each toasted bread slice, and return them to the oven. Bake until the cheese slightly melts and the bread edges are crispy, 2 to 3 minutes.

5. Add the dressing to the salad, and toss. Divide the salad evenly among 4 plates, and serve with 2 crostadas.