In a large skillet, heat 2 tbsp. oil over medium-high. Add the mushrooms; season with salt and pepper. Cook, stirring often, until browned, 5 to 8 minutes. Transfer to a bowl.
Add the remaining 1 tbsp. oil to the skillet. Add the onion and garlic. Cook over medium, stirring often, until softened, 5 minutes. Stir in the mushrooms, tomatoes, marsala, Marmite, if using, oregano and crushed red pepper. Cook, stirring occasionally, until the liquid is almost evaporated, 6 to 8 minutes. Stir the lentils into the ragu.
Meanwhile, in a large pot of boiling, salted water, cook the pasta to al dente. Drain, reserving 1/4 cup of the pasta cooking water.
Return the pasta to the pot and toss with the ragu and the reserved cooking water; season. Divide among bowls and top with the Parmesan and parsley.