Preheat the oven to 350°F and line a baking sheet with parchment paper.

Wash and trim the ends of the zucchini, then shred it on the fine side of a box grater.

Place the shredded zucchini in a clean kitchen towel or several paper towels and squeeze firmly over the sink until most of the liquid is removed and you have about 1 cup of tightly packed shreds.

In a medium mixing bowl, whisk together the peanut butter and egg until smooth and creamy.

Fold the shredded, squeezed zucchini into the peanut butter mixture until evenly combined.

Add the 2 cups of oat flour, ground flaxseed if using, and cinnamon if using, then stir with a sturdy spoon until a dough forms.

If the dough is sticky and clings to your hands, sprinkle in the extra oat flour 1 tablespoon at a time, mixing just until the dough is soft, pliable, and easy to handle.

Lightly dust a clean work surface with a small amount of oat flour and place the dough on top.

Roll the dough out to about 1/4 inch thickness, sprinkling a little more oat flour on the rolling pin if needed to prevent sticking.

Use cookie cutters or a knife to cut the dough into bone shapes, circles, or small squares.

Transfer the cut biscuits to the prepared baking sheet, placing them close together but not touching.

Gather and re-roll any dough scraps, cutting more biscuits until all the dough is used.

Bake the biscuits in the preheated oven for 25 to 30 minutes, or until the edges are dry and lightly golden.

Turn off the oven and, for crunchier treats, leave the biscuits inside with the door slightly ajar for an additional 10 to 15 minutes to dry.

Remove the baking sheet from the oven and let the biscuits cool completely on a wire rack before serving to your dog.