A silky, ultra-creamy tomato-cashew vodka sauce clings to al dente penne, layered with caramelized tomato paste, garlic, and a touch of heat—indistinguishable from the classic, yet entirely dairy-free.
Add cashews to a small heatproof bowl and pour boiling water over them; soak for at least 10 minutes while you cook.
Bring a large pot of salted water to a boil for the pasta.
Warm olive oil in a large pot over medium heat. Add diced onion and cook until golden at the edges, about 6 minutes.
Add sliced garlic and red pepper flakes; cook until fragrant, 1 to 2 minutes.
Add tomato paste and cook, stirring frequently, until dark red and beginning to stick to the pot, 8 to 10 minutes (reduce heat slightly if it sticks immediately; cook until the oil turns red).
Cook pasta in the boiling water until al dente according to package directions. Reserve 2 cups pasta water, then drain pasta.
Pour vodka into the tomato paste mixture and cook until the liquid evaporates, about 1 minute, scraping up any browned bits.
Transfer the tomato mixture to a high-speed blender with the soaked cashews and their soaking water; blend until completely smooth and creamy.
Return the blended sauce to the pot. Add salt and about 3/4 cup reserved pasta water; warm over medium-low heat until gently bubbling.
Add the drained pasta and vegan Parmesan; toss to coat, adding small splashes of additional pasta water as needed until the pasta is glossed with a luscious sauce.
Taste and adjust seasoning with more salt if needed. Garnish with more vegan Parmesan and serve.