Cook pasta in salted water until al dente. Drain and set aside.
While pasta is cooking, in a small bowl soak porcini mushrooms in 3/4 cup of warm water for 10 minutes until softened. Drain mushrooms, retaining the soaking water. Mince mushrooms and set aside.
In a large saucepan over medium-high heat, melt 1/4 cup of schmaltz or oil. Add onions, leek, mushrooms, salt and pepper. Saute until well-browned.
Add porcini mushrooms and garlic and cook for 2-3 minutes until garlic is fragrant and has softened. Add kasha, pepper, and thyme sprigs and cook for 3-5 minutes to toast the kasha. Stir in stock, cover, reduce heat, and simmer for 20 minutes.
Add cooked pasta to the pan and stir together. Garnish with parsley.