Bring a large pot of water to a boil and add 1 tablespoon salt.
Add the pasta and kasha and cook until the pasta is al dente and the kasha just tender, 10 to 12 minutes. Reserve 1 cup of the cooking water, drain, and return the pasta and kasha to the pot.
Meanwhile, heat the oil in a large skillet over medium heat. Add the onions and cook, stirring occasionally, until golden, 12 to 15 minutes.
Add the mushrooms and cook, stirring occasionally, until tender, 8 to 10 minutes.
Add the mushroom mixture to the drained pasta and kasha. Stir in the parsley, butter, reserved pasta water, ½ teaspoon salt, and ¼ teaspoon pepper.