Combine flour, flaxmeal, oats, sugar, salt, nutmeg, and soda in a large mixing bowl. In separate bowl, stir together milk, lemon, and sweet potato puree until smooth. Pour into dry ingredients and mix until you have a smooth batter. Heat waffle iron and spray lightly with cooking spray. Cook waffles according to manufacturer's instructions until golden and crispy on the edges, then remove and cover to keep warm until ready to serve. (With the size of my waffle iron I got about 9 waffles using 1/3 cup per.)