We had some mixed reviews on this dish in our household. I think that cooked buckwheat can sometimes have a bit of a slimy texture, similar to okra; some people like/don't mind this, while others aren't huge fans. That said, it has a nicely nutty flavor and light texture that make it an ideal base for more substantial counterparts.
Feel free and sub whatever is in season or in your refrigerator, though including something with a little bite (here, the red onion) with some cheese and nuts will really add another dimension to your bowl; you can also change your grain base to something like barley or quinoa if you're not stoked on buckwheat. In case you need inspiration for a variation, I think I'll be trying this again with fresh figs, arugula, goat cheese, and balsamic in a few weeks. :)
I'm going to give you an elemental breakdown of this recipe, since really this is just piling a bunch of stuff on top of some kind of grain in a bowl. I hereby give you creative license to throw in whatever feels right to you in any/all of the outlined categories, and I promise to post further winning combinations as we test them ourselves.
Cheese: sheep's milk feta
Roughage: arugula, sliced raw beets, apple/celery root/onion slaw (made by thinly slicing each ingredient and mixing with lemon juice and a tiny bit of oil), avocado