2 medium red beets (about 8 oz. total)
¼ cup fresh lemon juice
2 tablespoon prepared horseradish
1 tablespoon Sherry vinegar
½ cup extra-virgin olive oil
1 12-oz. skinless cod or pike fillet
5 tablespoon (or more) vegetable oil, divided
8 ounces smoked whitefish, skin and bones removed, flaked
3 large eggs, beaten to blend
1 large shallot, finely chopped
1 piece matzo, finely crushed (about ¼ cup)
¼ cup chopped fresh chives
2 tablespoon chopped fresh dill plus more for serving
Freshly ground black pepper
Preheat oven to 400°. Wrap beets individually in foil, place on a rimmed baking sheet, and roast until tender, 50–60 minutes. Let cool. Peel and cut into ¼” cubes.
Whisk lemon juice, horseradish, and vinegar in a medium bowl; gradually add oil, whisking to blend well. Season with salt.
DO AHEAD: Beets and vinaigrette can be made 1 day ahead. Cover separately and chill.
Heat oven to 400°. Place cod on a rimmed baking sheet. Rub with 1 Tbsp. oil. Roast until just opaque in center, 10–15 minutes. Let fish cool, then flake with a fork.
Mix cod, whitefish, eggs, shallot, matzo, chives, 2 Tbsp. dill, and 2 Tbsp. water in a medium bowl. Season with pepper and mix vigorously to combine (mixture will be stiff).
Reduce oven temperature to 200°; set a wire rack inside a large rimmed baking sheet. Heat remaining 4 Tbsp. oil in a large skillet over medium-high heat. Working in batches, drop scant ¼-cupfuls of fritter mixture into skillet; press gently with a spatula to flatten. Cook, turning once and adding more oil if needed, until golden brown and crispy around edges, about 4 minutes per batch. Transfer to prepared rack. Keep warm in oven while cooking remaining fritters.
Mix beets into vinaigrette. Serve fritters with vinaigrette; top with dill.
DO AHEAD: Fritter mixture can be made 1 day ahead; cover and chill.