Whisk, measure and sift together the flours, sugar, baking powder, baking soda and salt together.
Combine the milk with the oil, vanilla, and zest. Pour the wet mixture into the dry mixture and stir with a wooden spoon or spatula, just until a soft batter forms.
Using 2 tablespoons to form a round, carefully drop rounds of batter onto the simmering fruit mixture. Cover the saucepan with a tight-fitting lid or piece of aluminum foil and return to a simmer. Cook over low heat, without uncovering, for 25 minutes. The biscuits will feel firm to the touch and be slightly brown in color when they are cooked.
Using a pastry brush, lightly coat the top of the biscuits with maple syrup and/or sprinkle with cinnamon sugar, if desired.
Cool for 10 to 15 minutes before serving. Spoon a portion of biscuit and fruit into bowls and top with a scoop of natural flavored non-dairy ice cream, if desired.
Store any leftover slump covered with parchment paper at room temperature for up to 24 hours.