Steps:

Preheat the oven to 200°C (390°F).
Cook the lentils according to package instructions, then drain.
Sauté onion and garlic until translucent. Add mushrooms and cook until tender.
In a large bowl, combine cooked lentils, sautéed vegetables, walnuts, breadcrumbs, soy sauce, thyme, and rosemary. Season with salt and pepper.
Transfer the mixture to a greased loaf pan, pressing down firmly.
Meanwhile, toss the potatoes, parsnips, carrots, and Brussels sprouts in olive oil, salt, and pepper, and spread them out on a baking sheet.
Roast the vegetables in the oven alongside the lentil loaf, shaking the pan occasionally, until they are golden and tender, about 45 minutes.
Serve the sliced lentil loaf with the roasted vegetables and your favorite vegan gravy.

Vegan Roast Dinner