In a high-speed blender, combine the soaked cashews, roasted red pepper, harissa paste, olive oil, garlic, lemon juice, and salt. Add the water and blend until the sauce is completely smooth and creamy.
Add more water as needed to reach your desired consistency—thicker for a more decadent sauce, or thinner for a lighter, saucier version.
Taste the sauce and adjust the seasoning if necessary. Add more harissa paste for extra spice, more lemon juice for tang, or salt as needed.
Pour the cashew and red pepper sauce over the cooked pasta and stir well.
Serve the pasta with chill oil and chopped fresh basil (optional). Enjoy!