Rinse the lentils, cover with 750 ml (3 cups) of water and bring to a boil over high heat. Simmer over medium-low until tender, about 20 minutes. Drain very well and spread out to steam dry.
Mix the ground flaxseed with 3 tbsp of water and leave 5 minutes to thicken.
Heat 1 tsp of the oil in a skillet over medium heat and cook the diced onion and garlic until soft, about 5 minutes.
Preheat the oven to 200°C (400°F) and line a baking sheet with parchment.
Pulse the walnuts and oats in a food processor to coarse crumbs. Add the lentils, flax mixture, onion and garlic, soy sauce, tomato paste, smoked paprika and cumin and pulse to a coarse, sticky mix.
Shape into 6 patties about 2 cm (3/4 inch) thick, place on the tray and brush with the rest of the oil.
Bake for 20 minutes, flipping carefully at 10 minutes, until firm and browned.