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.
Serve on toasted buns with lettuce and tomato.
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