In a blender or food processor, combine raw cashews, nutritional yeast, Dijon mustard, and plant milk until smooth. Season with salt and pepper to taste.

In a large pot, bring water to a boil, adding salt to the water. Cook the spaghetti according to package instructions until al dente. Drain and set aside.

In a skillet, heat olive oil and vegan butter, then add chopped onion, minced garlic, and sliced mushrooms. Cook until the mushrooms are tender and the onions are translucent.

Add the drained spaghetti to the skillet with the sautéed vegetables and pour in the creamy sauce. Toss everything together over medium heat until heated through.

Finish with fresh parsley and additional ground black pepper if desired. Serve warm.