Bake the potatoes: Preheat the oven to 400°F. Scrub the potatoes clean and dry them thoroughly. Rub each potato with a little olive oil and sprinkle generously with flaky sea salt. Place directly on a baking sheet and bake for about 50–60 minutes, depending on their size.
Add the salmon: When the potatoes have about 12–15 minutes left to cook, arrange a small bed of fresh dill, parsley, or a combination of herbs on the baking sheet. Place the salmon fillets over the herbs, drizzle with olive oil, and season with flaky sea salt and black pepper.
Roast together: Return the baking sheet to the oven and cook until the potatoes are tender all the way through and the salmon flakes easily with a fork, about 12–15 minutes.
Make the sour cream mixture: While everything cooks, combine the crème fraîche, sour cream, dill, capers, lemon zest, and lemon juice in a small bowl. Season with salt and pepper to taste.
Fluff the potatoes: Remove the potatoes from the oven. Cut each potato lengthwise down the center, then make a few cuts across the middle. Gently squeeze the ends toward the center to open the potato, then use a fork to fluff the inside.
Assemble: Spoon a generous amount of the crème fraîche mixture into each potato. Flake the roasted salmon into large pieces and pile it over the top.
Finish and serve: Top with additional fresh dill and capers, squeeze fresh lemon juice over everything, and finish with more flaky sea salt and cracked black pepper if desired.