In a food processor, briefly whiz the pieces of smoked salmon with the thyme and lemon juice and zest. Scrape down the sides of the bowl, then add the butter and Boursin. Season lightly with the paprika, a touch of cayenne, and black pepper. Whiz again briefly until the mixture is combined, but still a little rough. Taste, and adjust the seasoning, salt, pepper or paprika. Chill for at least 20 minutes, longer if convenient. Remove from the refrigerator at least 10 minutes before serving.
Serve with or on slices of buttered rye bread, with a few watercress leaves.
Recipe courtesy of CUISINE GRAND-MERE (Time-Life Books, 2001) by Marie-Pierre Moine