In a large pot, boil salted water and cook linguine to al-dente according to instructions on pasta bag. Drain but do not rinse pasta, set aside. Reserve 1 cup pasta water.
In a large skillet or sauté pan, heat oil and butter over medium-low heat. Add diced onions, stirring frequently, until onions become translucent, about 2-3 minutes. Stir in garlic for 1 minute, then stir in cremini and king oyster mushrooms. Stir occasionally for 3-5 minutes until mushrooms start to brown.
Add 1 tablespoon flour, mix to coat the mushrooms and onion and garlic, about 1 minute.
Turn the heat up to medium-high. Add pasta water with the cooked linguine, stir for 2-3 minutes.
Add black truffles in oil, stir for 1 minute. Add parmesan cheese, stir until the sauce starts to thicken, about 2-3 minutes.
Remove from heat. Add salt, and pepper to taste. Stir in fresh parsley. Garnish with more parsley, parmesan, and pepper. Serve.