Put the flaxseed into a glass with the water and set aside to thicken. Preheat the oven to 200 degrees C (400F). Line a loaf tin with greased parchment paper, and set aside.
Wash the lentils well and transfer them to a medium-sized saucepan with the the vegetable stock. Bring to a hard boil, reduce the heat and simmer over low heat until the water has been absorbed and the lentils are cooked, about 25-30 minutes. Drain off any excess water (there should not be much.)
Heat the olive oil in a saute pan and add the onions, carrots, celery, garlic, rosemary, cayenne, mace and salt. Saute until soft (5-8 minutes.)
Toast the sunflower seeds in a heavy bottomed pan, moving them around for a few minutes until they release their aroma. Make sure they don't burn. Transfer the seeds to a food processor along with the rolled oats and pulse until they are the consistency of a course meal.
Add half the lentils, rolled oats, and sunflower seeds to the pan with the vegetables, and mash the mixture with a masher or a wooden spoon.
Add the reserved whole lentils to the mixture and stir gently. Adjust the seasoning.
Pack the lentil mixture into the prepared loaf tin with the back of a wooden spoon.
Mix together the ketchup, balsamic vinegar, maple syrup and onion powder. Spread it over the lentil loaf and bake for 40-50 minutes.