Caramelize the onion: Heat a generous drizzle of olive oil in a wide, large pan over medium-low heat. Add the diced onion and a pinch of salt. Cook slowly, stirring often, until deeply golden and caramelized, about 15 minutes.
Add garlic: Increase heat to medium. Stir in the minced garlic and cook for another minute, just until fragrant.
Cook the zucchini: Add the grated zucchini straight into the pan and add a pinch of salt. Stir occasionally and let it simmer for 10 - 13 minutes, or until the zucchini softens and most of the water has cooked off.
Season and finish: Add black pepper, lemon juice, and fresh basil. Cook for another 2 minutes, then remove from heat and allow to cool down for about 5 minutes.
Blend it up: Carefully transfer the mixture to a blender (or use an immersion blender in the pan) and blend until smooth and creamy. Taste and adjust seasoning as needed.
Toss with hot pasta: Return the sauce to the pan if needed and toss with cooked pasta. Add a splash of pasta water to loosen the sauce if necessary. And add more olive oil or vegan butter (optional)
Serve: Garnish with more fresh basil, lemon zest, or a sprinkle of vegan parmesan.