Preheat oven to 350 degrees. Line a large baking sheet with parchment paper and set aside.

Place the grated apple onto a layer of paper towels and squeeze most of the liquid out of it. Set apple aside.

Combine the flours and ground flax in a medium mixing bowl. Add the water and peanut butter, stirring until combined—the dough will be pretty dry so I recommend mixing with your hands instead of a spoon or spatula. Mix in the grated and squeezed-dry apple until evenly distributed. If it’s too dry add a bit more water; if it’s too wet add a bit more whole wheat flour. Knead until the dough becomes a cohesive mass.

Roll dough out to 1/4-inch thickness on a lightly floured work surface. Cut into 1-inch hearts (or other shapes) and bake 26 to 30 minutes, until treats are light golden brown. (Larger cookies will take longer to bake, smaller ones will take less time so adjust baking time accordingly.) Remove from the oven and allow to cool completely before serving or storing in an airtight container.