Scrub lemon. Grate zest. Combine in bowl with butter and honey and whip with wooden spoon until pale and fluffy. Halve lemon and squeeze 1 tablespoon juice. Whip into butter, a teaspoon at a time.

With sharp bread knife, slice crust from all sides of the loaf, trimming to make a neat rectangle. Cut lengthwise into slices about 1/4 inch thick (8 long rectangles). Lay 4 slices side by side on each of two cutting boards. Scoop a tablespoon of butter onto each slice. With flexible spatula, spread evenly to edges. Chill.

Quarter onion, cutting through "poles." Cut lengthwise into thin slices. Arrange 4 slices on each long bread slice to nearly cover, leaving a little space between onions. Sprinkle a stripe of dill down the center of each long slice. Press bread rectangles tightly together to make a single-layer rectangle (about 16 by 7 inches). Cover each with wax paper. Press down with a rolling pin to flatter slightly. Chill until serving time.

To serve, grind over pepper and sprinkle with salt. Cut each long slice into 4 sections, cutting between onion for tidiness. Halve each if you like small canapes. Serve at once, so the salt remains crunchy.