In a heavy dry pan over medium heat, toast the onion and scallions until they are browned in spots here and there. Add the garlic and the ground cumin and fennel and continue toasting till fragrant; a few more seconds. Remove from heat. In a blender, blend together the avocado, buttermilk, mayonnaise, anchovy, salt, sugar, white pepper and mustard powder until smooth.
Add the contents of the pan to the blender along with the poppy, sesame and charnushka seeds. Buzz just to combine, or, if you would like a chunkier result, scrape the blender contents into a bowl and blend in by hand, tasting as you go. Add the fresh or dried dill. Keeps well in a jar, referigerated of course, for a couple of weeks.
If you stock buttermilk, you can do worse than to make up a ranch dressing as you feel like it, either from scratch or with a nice spice mix like the one from Penzey's, which you bloom first in a bit of water and then add to half and half buttermilk and mayonnaise. You can also take that same half and half mix and add some leftover pesto sauce or guacamole to make a quick, light salad dressing. Avocado, buttermilk and scallions were the staples of what we used to call Green Goddess dressing.
You can also get curious along the lines of the Everything Bagel theme and make this Green Goddess of Everything dressing, which did come out well, though there are some that thought all the whole seeds were a bit much. I might try leaving out the poppy seeds sometime and pestling up the sesame and nigella with a pinch of sugar into something I think might be like a kind of smoky halvah, and add that to the dry pan with the garlic.
Charnushka is great stuff, if you've never had it. Of course you can leave it out of this recipe, but track some down, particularly if you're a bread baker. It's a smokier and more potent sprinkle than poppy seeds.