Start by preparing all your ingredients. Chop all the vegetables and rinse the buckwheat.
Heat the oil in a large pot over medium heat. Once the oil is shimmering, add the mushrooms and onions. Sprinkle with a pinch of salt and stir to coat the vegetables evenly in oil. Cook, stirring regularly, until the onions are translucent and the mushrooms have released all their water and are beginning to brown—about 10 minutes. Reduce heat to medium-low if the vegetables are browning too quickly.
Add a bit more oil to the pot, if needed, and then stir in the carrots, celery, garlic, rosemary, and thyme. Stir frequently and cook for about 1 minute, until the garlic is fragrant.
Stir in the buckwheat and stock. Increase the heat to high and bring to a boil. Reduce heat to a simmer, partially cover with a lid, and cook for 15-20 minutes, until the buckwheat is tender but not mushy.
Remove from the heat and stir in the apple cider vinegar and dulse flakes. Taste and adjust the salt and pepper, as desired. Enjoy!
Store leftovers in the fridge for up to 4 days or in the freezer for up to 3 months.