Preheat the oven to 180°C (160°C fan).
In a pan, heat olive oil over medium heat. Sauté onions and garlic until softened.
In a large bowl, combine sautéed onions and garlic with drained lentils, chopped walnuts, breadcrumbs, flax seeds, tomato puree, soy sauce, dried thyme, dried rosemary, salt, and black pepper. Mix well to combine.
Transfer the mixture into a greased loaf tin, pressing it down firmly. Smooth the top.
For the cranberry glaze, combine fresh cranberries, brown sugar, balsamic vinegar, Dijon mustard, ground cinnamon, orange zest, and water in a saucepan. Simmer over medium heat, stirring occasionally, until the cranberries burst and the mixture thickens into a glaze-like consistency. Remove from heat.
Place the lentil walnut loaf in the preheated oven and bake for 30-35 minutes until it's firm and slightly crispy on the edges.
During the last 15 minutes of baking, generously brush most of the cranberry glaze over the loaf, allowing it to caramelize. Decorate the loaf with thinly sliced carrots for the last 5 minutes of baking (optional).
Once baked, let the loaf cool for a few minutes before slicing with a sharp, serrated knife.
Serve slices of the lentil walnut loaf with a drizzle of remaining cranberry glaze and festive trimmings, such as roasted vegetables or a side of mashed potatoes.