1. Warm a medium pan over a low heat. Tip in the buckwheat. Toast for 4-5 mins, stirring often, till browned and nutty. Tip into a processor or blender and blitz to grind, or use a pestle and mortar or a bowl and the bottom of a jar to finely crush it.
2. Crumble the stock cube into the pan. Pour in 700ml boiling water and stir to mix. Pour in 150ml milk and bring to a gentle simmer. When it's just bubbling, whisk in the polenta and the ground buckwheat, making sure you whisk as you go so the polenta and buckwheat combine smoothly.
3. Add a pinch of salt. Pop a lid on the pan, turn the heat to low and cook for 20 mins, stirring a few times, till the polenta is thick.
4. Meanwhile, peel and finely chop the onion. Put a large frying pan on a medium heat and add ½ tbsp oil, the onion and a pinch of salt and pepper. Cook for 5 mins, stirring every so often, till translucent.
5. While the onion fries, thickly slice the mushrooms. After the onion has cooked for 5 mins, stir in the mushrooms. Cook, stirring often, for 8 mins till they are browned and juicy.
6. While the mushrooms cook, peel and crush the garlic. Finely chop the chard stalks and pull the thyme leaves off their stalks. Add the garlic and chard stalks to the mushrooms. Cook and stir for 2-3 mins till the pan smells sweet and aromatic. Add the chard leaves and most of the thyme. Stir till the chard leaves wilt. Take the pan off the heat.
7. Chop the butter into chunks and add to the polenta. Stir to melt it in. Taste and add a pinch of salt if needed. Spoon the polenta into warm bowls and top with the mushrooms and chard.
8. Wipe the frying pan clean and add 1 tbsp oil to it. Set the pan over a medium-high heat. Crack in the eggs, pop a lid on (or cover with a baking tray) and fry for 2-4 mins till the yolks are set to your liking.
9. Lay a fried egg on top of each bowl. Garnish with the remaining thyme leaves and sprinkle with a little sumach to serve.