Tip the porcini mushrooms into a heatproof bowl. Cover with 200ml boiling water and set aside. Peel the shallots (keep them whole). Finely slice the leek and celery. Peel and slice the carrot into diagonal 1cm chunks.
Thickly slice the white and shiitake mushrooms, leaving any smaller ones whole. Peel and thinly slice 1 of the garlic cloves.
Heat 2 tsp oil in a large pan over a medium heat. Add the whole shallots to the pan. Cook for 3 mins till browned. Add in the leek, celery and carrots. Season. Cook, stirring, for 3 mins.
Add in the sliced mushrooms and garlic and fold through the veg. Cook for 5 mins. Pour in the porcini mushrooms along with their soaking liquid. Let it bubble up.
Tip in the chopped tomatoes. Add the thyme leaves and stir. Cover and simmer for 15 mins. While the bourguignon is simmering, tip the buckwheat into a pan. Cover the buckwheat with boiling water. Cover and simmer over a medium heat for 15 mins.
Peel and grate or crush the remaining garlic clove. Finely chop the parsley stalks and leaves. Zest and
juice the lemon. Whisk everything together in a small bowl with 1 tbsp water and 1 tbsp oil. Season. This is your gremolata.
After the bourguignon has cooked for 15 mins, taste and adjust the seasoning. Serve in warm bowls with the buckwheat, and the gremolata drizzled on top.