Preheat your oven to 350 degrees. In a small bowl, combine the ground flax and 1/2 cup water. Set aside. Simmer the lentils, broth and a pinch of salt for about 25 minutes or until the lentils are tender and have absorbed all the broth.
Meanwhile, prepare your veggies. Chop the onion, grate the carrot and dice the celery. Saute the onion and celery in the olive oil over medium high heat for six minutes, or until tender, not caramelized. Add the garlic and carrot and cook for about another four minutes.
Toast the walnuts in the oven for six to seven minutes. Add them to the onion/carrot/garlic mixture on the stove and stir well. Add the oregano, salt and pepper as well and then take the mixture off the stove and transfer to a large bowl. Add the breadcrumbs, flax/water and cooked lentils and toss well. Press mixture into a greased loaf pan and set aside while you make the topping.
In another small bowl combine the ketchup, maple syrup and vinegar. Spread on top of loaf. Bake for 40 minutes and serve.