1. Preheat the oven to 350°F (180°C).
2. Bring 1 cup (250 mL) of water to a boil in a small saucepan; add the lentils and cook for 12 minutes, or until very soft. Drain well and set aside to cool slightly.
3. Into the bowl of a food processor, place the butter, sugar, egg, and lemon zest and juice. Add the lentils and pulse until well blended and smooth. It may look curdled–that is fine.
4. Add the flour, baking powder, and salt and pulse until the dough begins to come together. Add the poppy seeds and pulse once or twice, then carefully remove the dough and knead it once or twice with your hands. If it is really sticky, do this on a well-floured surface, incorporating a little more flour into the dough.
5. Place on a parchment-lined baking sheet and with dampened hands, shape into 12-14 inch (31-36 cm) long log, and then flatten to 3-4 inches (8-10 cm) wide. Bake for 25-35 minutes, or until pale golden and set. Remove from oven and let cool completely.
6. Reduce the oven to 275°F (150°C). Once the biscotti log has completely cooled, slice on a slight diagonal into ½-inch thick slices and return to the baking sheet, sitting them upright, spaced at least one inch (2.5 cm) apart. Return to the oven for 30 minutes, or until crisp, pale golden and dry.
7. Whisk together the confectioner’s sugar, cream, and lemon juice. Drizzle over the cooled biscotti and leave until set.