Heat oil in a 2-quart saucepan. Add onion, garlic, and mushrooms and sauté until onion is soft and most of the liquid is evaporated, 10–15 minutes. Add potatoes to onions, stir, and sauté 2 minutes. Add kasha to mixture and stir. Pour in boiling water, salt, and pepper. Turn heat to low. Cover pot and simmer 15 minutes.