~----------For decoration:--------- ~----------For the dill sauce:------ POACH THE SALMON: To time the poaching, measure the fish and allow 10 minutes for each inch of thickness; begin timing when the liquid starts to simmer. To check for doneness, insert a metal skewer into the thick flesh behind the gill. It should slide in easily. A six-pound salmon needs either a fish poacher or a roasting pan fitted with a rack at the bottom for easy fish lifting. I use a roaster. It usually means the fish has been curved to fit in, and the benefit of this is that it retains this jaunty pose once cooked so it fits more easily onto a platter. Chill it carefully, then unwrap it, and gently pull off the skin. If the fat underneath the skin is gray, scrape it away with a knife to reveal the orange flesh. Cut down the center of the fish to the backbone, and then ease one half of the fillet from the bones. Gently pull off the flesh, then remove the backbone and replace the fish. This way it is easier to serve at a buffet. Peel and halve the cucumbers. Scrape out the seeds, and then thinly slice them. Arrange cucumber on top of the salmon in a pattern resembling fish scales. Slice a lemon, and arrange around the fish. Chill well. For the sauce, combine the sour cream with the mayonnaise, and whisk until smooth. Add the onion, lemon juice, dill, salt and pepper. Serves 10 to 12, or more if there are other entrees. Note: The fish can be cooked up to a day in advance and refrigerated, tightly covered. The sauce can be made up to two days in advance and refrigerated.