Preheat oven to 400 degrees F. Coat a baking sheet with cooking spray.

Whisk egg, flour, scallions, dill, horseradish, salt and pepper in a large bowl. Stir in vegetables and bacon (if using).

Heat 2 teaspoons oil in a large nonstick skillet over medium heat. Cook 4 pancakes per batch: place about 1/4 cup vegetable mixture in a little of the oil and press with the back of a spatula to flatten into a 2- to 3-inch pancake. Cook until crispy and golden, 1 1/2 to 3 minutes per side. Transfer the pancakes to the prepared baking sheet. Continue with 2 more batches, using the remaining 4 teaspoons oil and vegetable mixture. Transfer the baking sheet to the oven and bake for 15 minutes. Serve garnished with sour cream, if desired.

Beets, carrots and parsnips are easily peeled with a vegetable peeler, but for tougher-skinned roots like celeriac, rutabaga and turnips, removing the peel with a knife can be easier. Cut off one end of the root to create a flat surface to keep it steady on the cutting board. Follow the contour of the vegetable with your knife. If you use a vegetable peeler on the tougher roots, peel around each vegetable at least three times to ensure all the fibrous skin has been removed.