Great article from T. Colin Campbell's Center for Nutrition Studies --
https://nutritionstudies.org/how-to-build-the-ultimate-plant-based-burger
Link is from their free newsletter. Signup is at the bottom of the page. Recommended.
The article mentions mushroom powder. Mushrooms are one of the key sources of vegan umami.
https://www.onegreenplanet.org/vegan-food/how-to-add-umami-flavor-to-vegan-dishes/
(Think mushroom gravy on mashed potatoes.)
It turns out that one can make DIY mushroom powder or order it online. Something I want to try.