Cook the lentils in boiling water according to the package instructions. Drain in a sieve, rinse with cold water and allow to drain. Finely chop the onion and garlic. Heat 1 tablespoon of oil in a pan and sauté the onion and garlic over medium heat for about 5 minutes. Remove from heat.
Grind the oat flakes and linseed into a fine flour in a speed chopper/food processor. Rinse and drain the beans in a colander. Add the lentils, onions, garlic, walnuts, sunflower seeds and soy sauce to the food processor/speed chopper and puree until lightly chunky.
Clean, peel and finely grate the carrots. Clean and finely chop the mushrooms. Rinse the parsley, shake dry, pluck the leaves from the stems and chop finely. Mix the lentil and bean mixture well in a large bowl with carrots, mushrooms, parsley, 2 tablespoons of tomato paste, mustard and roast seasoning. Pour the mixture evenly into a loaf pan (approx. 20 x 11 cm) lined with baking paper and smooth it out. Bake the roast in a preheated oven at 180 degrees (fan oven not recommended) on the second rack from the bottom for about 30 minutes.
Meanwhile, finely dice the shallots and garlic. Rinse, clean, peel and finely dice the celery, parsley root and carrot. Clean and finely chop the mushrooms. Heat rapeseed oil in a pot. Fry the vegetables, cloves, crushed allspice seeds, juniper berries and bay leaf on all sides until light brown, about 5 minutes.
Deglaze with red wine and let it reduce to ⅓ for about 10 minutes, stirring occasionally. Fill with vegetable broth, bring to the boil and simmer uncovered over medium heat for about 20 minutes. Puree the sauce finely with a hand blender and strain through a sieve into another pot. Bring the sauce to the boil and thicken slightly with the starch dissolved in a little cold water. Season the gravy with salt, pepper and cranberries. Keep warm until serving.
Mix the cranberries and remaining tomato paste (1 tablespoon) until smooth, season with salt and pepper. Brush the roast with it after 30 minutes of baking and bake for another 20 minutes. Remove from oven and let cool for 15 minutes. Remove the roast from the tin along with the baking paper and cut into 1-2 cm wide slices.
Meanwhile, peel the potatoes, rinse them, cut them roughly and cook them in plenty of boiling salted water for 15-20 minutes until soft. Drain and let steam for 2 minutes. Heat oat milk and vegan butter in a saucepan. Peel and finely grate horseradish. Press potatoes through a potato press or mash with a potato masher. Add the milk-butter mixture and about ⅔ of the horseradish to the potatoes and stir until smooth. Season with nutmeg and salt. Keep warm.
To prepare the roast, rinse the chervil, shake it dry and pluck the leaves from the stems. Heat the remaining rapeseed oil (1 tbsp) in a non-stick pan and fry the roast slices over medium heat for 2-3 minutes on each side. Serve the roast with puree and sauce. Sprinkle with chervil, the remaining horseradish and some pepper and serve.