In a food processor, whirl cheese, oil, lemon zest and juice, 1/2 tsp. salt, and 1/4 tsp. pepper until puréed.
Remove tops and tough outer layers from fennel. Cut bulb in half lengthwise, core, and thinly slice crosswise.
In a large bowl, toss pears, fennel, arugula, and hazelnuts. Drizzle salad with as much dressing as you like and toss until evenly coated. Season to taste with salt and pepper, and serve more dressing on the side.