Sauteed shallots and mushrooms are combined with tangy creme fraiche, piquant horseradish, and fresh dill to create a richly flavored sauce for pasta.
Bring a large pot of salted water to a boil. Add pasta and cook until slightly less than al dente, about 2 minutes less than package directions. Reserve 1 cup pasta water, then drain.
Meanwhile, heat oil in a large straight-sided skillet over medium-high. Add shallots; cook, stirring, until translucent, about 2 minutes. Add mushrooms, season with salt and pepper, and cook until tender and browning in spots, 7 to 9 minutes. Stir in horseradish, pasta, and 1/2 cup pasta water; cook, stirring and shaking pan until pasta is cooked and sauce begins to coat pasta, 1 to 2 minutes. Remove from heat and stir in creme fraiche and chopped dill; continue stirring and shaking pan, adding more pasta water as needed to create a creamy sauce. Serve, garnished with more creme fraiche, dill sprigs, and pepper.