For the meatloaf: Combine the water and buckwheat in a large saucepan with 1 tsp salt. Bring to the boil and then reduce to a simmer for about 40-50 minutes until the buckwheat is very tender.
Drain and set aside to cool. Once cool, combine in a food processor with the walnuts and pulse a few times to break down.
Preheat the oven to 180°C (160° fan) | 350F | gas 4. Grease and line the base and sides of a 900 g | 2 lb loaf tin with grease-proof paper.
Heat the olive oil in a large saucepan or casserole dish set over medium heat. Add the red onion and a pinch of salt, sweating for 5-6 minutes until softened.
Add the garlic salt, dried herbs, and a little pepper. Stir well to combine before removing from the heat.
Stir in the buckwheat wheat mixture and beaten egg. Season with a little more salt and pepper.
Spoon the mixture into the prepared loaf tin, smoothing the top. Bake for about 1 hour 5-15 minutes until golden-brown on top and dry to the touch; a cake tester should come out clean from its centre. Remove to a wire rack to cool.
For the sauce: Whisk together all the ingredients for the sauce in a small mixing bowl. Season to taste with salt and pepper.
To serve: Once cool, turn out the loaf from the tin and cut into slices. Drizzle with the sauce and serve with a garnish of walnuts and coriander sprigs.