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.
Recipe courtesy of Elizabeth Schneider's VEGETABLES FROM AMARANTH TO ZUCCHINI (Morrow, 2001)