This comforting recipe is a BLD, which means it's great for breakfast, lunch, or dinner.
Preheat the oven to 225°. Place the potatoes in a strainer; season with salt. Let stand for 10 minutes, then wring or press out any excess liquid. Transfer the potatoes to a bowl. Add the parsnip, scallions, eggs, and flour; season with pepper. Mix well.
In a large skillet, heat about 1/8 inch of oil over medium to medium-high. When the oil begins to ripple, add four 1/2-cup mounds of the potato mixture to the skillet, spacing the mounds evenly apart. Using a fork, flatten the mounds into patties. Cook the cakes until deep golden, about 5 minutes per side. Transfer to a wire rack set inside a baking sheet to drain; place in the oven to keep warm. Repeat with the remaining potato mixture for a total of 8 cakes.
In a small bowl, whisk the crème fraîche, herbs, horseradish, and lemon juice for the sauce; season.
To serve, divide the parsnip-potato cakes, smoked salmon, cucumber, and onion among 4 plates. Top with a fat dollop (or two) of the sauce and capers.