Soak the cashews overnight. If you’re in a hurry, soak them in hot water for at least 1 hour. The day after, drain and rinse. Blend cashews with water until smooth and set aside.
Cook the pasta according to package directions. Drain and set aside.
Meanwhile, heat oil in a large skillet over medium-high heat. Sauté the onion until it's soft and lightly browned, for about 3-4 minutes.
Add garlic and mushrooms, stir, and sauté until they become soft and golden brown, for about 5-8 minutes.
Add the flour and cook over medium heat for 1-2 minutes, stirring frequently.
Gradually whisk in the vegetable stock until the mixture is smooth without any lumps.
Add the soy sauce (or vegan Worcestershire sauce) and mustard and give it a good stir.
Bring the mixture to a boil, then reduce the heat to medium and continue cooking for approximately 3-5 minutes or until it thickens, making sure to whisk frequently.
Add the cashew mixture, salt, and pepper and stir until well combined.
Stir in the cooked pasta and heat it through for 1-2 minutes until warmed. Serve immediately.