For The Relish
2 tablespoons extra-virgin olive oil
2 teaspoons white balsamic vinegar
1 teaspoon fresh lemon juice
¼ teaspoon kosher salt
¼ teaspoon freshly ground black pepper
2 tablespoons finely chopped red onion
1 tablespoon finely chopped fennel fronds
1 ¼ cups ¼-inch-dice fennel bulb (cored; from 1 medium)
1 ¼ cups ¼-inch-dice Bosc or Anjou pear (red, if available; from 1 medium)
For The Latkes
2 cups ½-inch cubes crustless day-old French bread
6 cups coarsely grated peeled trimmed celery root (from 2 grapefruit-sized)
¼ cup chopped fennel fronds
1 ½ teaspoons baking powder
1 teaspoon (packed) finely grated lemon peel
¾ teaspoon kosher salt
2 large eggs, beaten to blend
¼ cup (or more) grapeseed oil for frying
Sour cream or crème fraîche (optional)
To make the relish: In a medium bowl, whisk the oil, vinegar, lemon juice, salt, and pepper to blend. Stir in the onion and fennel fronds, then the diced fennel. (Can be made 2 hours ahead. Let stand at room temperature, tossing occasionally.) Mix in the diced pear and serve
To make the latkes: Place a large rimmed baking sheet on the center rack of the oven and preheat to 325 degrees. Grind the bread cubes in a mini processor until reduced to small crumbs. Place the grated celery root in a large bowl. Add the fennel fronds, baking powder, lemon peel, and salt. Toss until the seasonings are evenly distributed. Mix in the eggs, then 1 cup of breadcrumbs. Toss until the batter holds together. Lay a large piece of foil on a work surface. Fill a ⅓-cup measuring cup with batter. Turn the cup over and, tapping the edge sharply on the foil, turn out the batter in a mound. Repeat 3 more times for 4 latkes.
Add ¼ cup oil to a large nonstick skillet. Heat over medium heat for 3 minutes. Using a metal spatula, transfer 1 mound of batter to the skillet. Gently flatten to a 3-inch-diameter round (latke will be thick). Repeat 3 more times for 4 latkes. Cook until deep brown and crisp on the bottom, pressing stray shreds of celery root back to the latkes, 3 to 4 minutes. Turn the latkes over. Cook until deep brown and crisp on the bottom, 3 to 4 minutes. Transfer the latkes to the baking sheet in the oven to keep warm. Repeat with the remaining batter, adding more oil to the skillet by tablespoonfuls as needed and making at least 12 latkes.
Divide the latkes among plates. Spoon relish alongside. Serve with sour cream or crème fraîche, if desired.