Preheat the oven to 175°C/350°F and line a loaf tin with baking parchment.
Mix the ground flax and water together in a small bowl and set aside.
Add the peanuts, walnuts, hazelnuts and sunflower seeds to a blender or food processor and blend until they become coarse flour.
Transfer to a large mixing bowl and add the mixed herbs, sage, pepper and brown rice flour, and stir until combined.
Chop the onion, carrot and celery into four large chunks each. Place them all in a blender or food processor and blend until they are finely chopped.
Add the olive oil to a saucepan and heat. Add the onion, carrot and celery and fry the mixture gently for about 10 minutes, until the vegetables are softened and fragrant.
Transfer the vegetables to the large mixing bowl with the dry ingredients, add the flax egg and tamari, and mix everything together really well.
Season to taste with sea salt (I would sample the mixture at this point before adding salt as soy sauces tend to vary in their saltiness, and you may not need to add any extra!).
Pour the mixture into your loaf tin and press it down to flatten it with a spatula.
Press the ¼ cup of nuts your reserved for decoration into the top of the roast, placing them evenly all over the surface.
Bake the roast in the oven for approximately 45 minutes, until the top of the loaf is browned.
Remove from the loaf tin and place on a cooling rack for 30 minutes - or serve immediately!
Serve with roast potatoes, parsnips, carrots, kale and gravy.