Heat the oil or butter in a large pot. Add onion, garlic and salt, sautéing them until the onions are soft. Once the onions are ready, add in the potatoes and mushrooms. Cook for a few more minutes, covered, until the mushrooms have released their juices and the whole mixture is lovely and moist. Add kasha to the mix and stir, coating the grains well. Pour in the boiling water and turn down the heat to low. Cover and simmer 15 minutes until all water is absorbed into the grains. When finished, remove the lid and allow the kasha to rest for about 5 minutes. Fluff and garnish when serving!