Pat the roast dry with paper towels, and set it bone-side up on a baking sheet.
In a large bowl, whisk together the salt, peppercorns, dry mustard, and garlic powder. Add 3⁄4 cup warm water and stir until the salt is evenly sandy and begins to clump up a little. Using your hands, spread about one-third of the salt mixture over the small strip of meat next to the rib bones and pack it into a firm, even layer. Carefully turn the roast over and spread the remaining seasoning over the meat and fat, leaving the meaty ends of the roast clean. (It’s fine if some of the salt falls off; just do the best you can to create a shell of salt over the entire top of the roast.) Gently rub the exposed ends with your salty hands—you’re just seasoning the meat, so you don’t want a full crust here. Set aside and let sit at room temperature for 1 hour before roasting.
Meanwhile, make the crème fraîche: In a small mixing bowl, stir together the store-bought crème fraîche, sour cream, horseradish to taste, chives, and lemon zest and juice. Season to taste with salt and pepper, cover, and refrigerate until ready to serve, or up to 3 days.
Preheat the oven to 450°F and arrange a rack in the lower third of the oven.
Roast the meat for 1 hour, until the salt crust is nicely browned. Reduce the temperature to 300°F, and roast for 2 1⁄4–3 1⁄2 hours more (up to 4 hours more for a full 7-rib roast), until it registers 135°F on an instant-read thermometer inserted into the center of the roast for medium-rare. (The top of the roast will be very dark.)
When the meat is done, remove it from the oven and let it rest for at least 30 minutes, or up to 1 hour. To serve, remove the crust—it should come off in a few big pieces—and gently transfer the meat to a large cutting board, bone sides up. Slice into large slabs between the bones, or carve the meat away from the bones, and cut the resulting roast into thin slices. Serve warm, with the crème fraîche alongside.