If you are using dried pasta put it to cook now and follow the timing on the packet. To make the sauce, melt the butter and gently soften the onion in it until it is translucent. Add the garlic and cook for 1-2 minutes more. Stir in the horseradish, capers and cream. Season and let the sauce simmer very gently. If you have fresh pasta cook it now. While it is in the pan, stir the lemon juice into the sauce and keep stirring so that it thickens slightly. Add the smoked trout and let it heat through. At the last moment, stir in the dill. When the pasta is cooked al dente, drain well and turn it into a warmed dish, toss with the sauce and serve.