Mix flax seed and warm water in a small bowl. Stir then let sit for about 2 min. Mix lemon zest, agave nectar, flax seed mixture, milk, and margarine until combined. Add flour, baking powder, and salt. Mix until incorporated, but do not over mix. Pipe into greased donut baking pans about half full (they puff up a lot) Bake for 12-14 minutes at 350F until golden. 12 minutes if using a mini-donut pan, 14 if using a regular-sized donut pan. Remove from oven, let sit in the pan for about 1 min, then remove from pan and let cool on a wire rack until cooled. Mix lemon juice (strained) and powdered sugar until all lumps are broken up. Dip cooled donuts in glaze, let excess drip off, then let them dry on the wire rack. Leave for about 10-15 minutes until dry, then eat!