How to Make It
Combine onion, cashews, garlic, and 2 1/2 cups of the broth in a medium saucepan. Bring to a simmer over medium; simmer, stirring occasionally, until cashews are tender, mixture is reduced, and liquid is completely evaporated, about 20 minutes. (Don’t let onion brown.)
Place cashew mixture, yeast, lemon juice, pepper, 1 teaspoon of the salt, and remaining 1/2 cup broth in a high-powered blender. Process on medium-high speed until well blended, about 2 minutes.
Cook pasta according to package directions; reserve 1 cup cooking water. Drain and rinse pasta with hot water.
Heat oil in a large skillet over medium-high. Add cashew mixture, reserved pasta water, and remaining 1/4 teaspoon salt; bring to a simmer. Remove from heat. Add pasta, and toss to coat. Sprinkle with parsley, and serve immediately.