Bring a large pot of salted water to a boil. As it heats, begin cooking the mushrooms. Add the pasta to the boiling water once mushroom cooking has started. Cook pasta according to the packet directions, subtracting one minute from the suggested time. Before draining, reserve a mug of the pasta cooking water.
In a wide skillet, melt half the butter together with the olive oil over medium heat. Add the sliced mushrooms and cook until their released moisture has evaporated and they begin browning on the edges, about 5 minutes. Sprinkle in the salt and pepper midway through cooking.
Incorporate the garlic and the remaining butter, continuing to sauté for another 2 minutes until both the mushrooms and garlic take on a golden hue.
Toss the drained pasta into the pan along with approximately 3/4 cup of the reserved cooking liquid and the parmesan. Stir gently until the liquid reduces to a glossy coating. If the sauce becomes too dry, stir in additional pasta water until desired consistency is achieved.
Adjust seasoning to taste. Remove from the heat and serve promptly, finished with a sprinkling of fresh parsley and additional parmesan.