Hawthorn berries were once used as a substitute for flour when baking bread. Ignorantly, I dried them and ground them into flour with a coffee grinder. That worked. I substituted only 3 heaping tablespoons of the resulting flour at a time from 400 grams of whole wheat flour, though.
Later I read (where I unfortunately do not remember) that the hawthorn berries are cooked and then passed through a sieve. The resulting pulp is dried and then finely ground in a blender. (I have not yet tried to do this myself). However, I find this preparation quite energy-intensive, especially because it is more difficult to dry in the air in autumn.