Preheat the oven to 325°F. Line all 12 cups of a standard muffin pan with unbleached parchment liners.

Whisk the prepared nut flour, prepared ground flax, homemade coconut flour, mineral salt, and cinnamon in a medium bowl. Break up every coconut-flour lump; the mixture should look loose and evenly speckled.

Blend the dates, coconut oil, prepared nut or seed butter, finely chopped apple, and eggs in a food processor. Stop and scrape the sides once, then blend until the mixture is thick, glossy, and tan with no separate egg or oil streaks.

Add the dry mixture to the wet mixture. Mix thoroughly until no dry flour remains. The batter should mound on a spoon and settle slowly rather than pour.

Fold in the fresh blueberries gently. Fold in the optional prepared walnuts at the same time if using.

Divide the thick batter evenly among the 12 lined cups. Smooth any sharp peaks with the back of a spoon.

Bake for 20 to 25 minutes, until the tops are modestly puffed, browned, and dry rather than shiny. A toothpick inserted through the crumb instead of a blueberry should come out clean.

Rest the muffins in the pan for 10 minutes, then move them to a cooling rack so the moist crumb can finish setting.