Cook the pasta: Cook the spaghetti or other pasta in salted water according to the package instructions.
Prepare the avocado sauce: While the pasta is cooking, add the avocados, garlic, olive oil, lime juice, nutritional yeast, and herbs to a food processor or small blender.
Add pasta water and blend: Shortly before the pasta is ready, reserve 6 tablespoons of the cooking water and add it to the sauce ingredients. Blend until smooth and creamy, then season with salt and black pepper to taste.
Toss the pasta with the sauce: Drain the pasta, return it to the still-hot pot, and immediately toss it with the avocado sauce. Don't put the pot back on a hot burner – the sauce should only be warmed by the residual heat of the pasta. If you like, stir in the chopped tomatoes as well.
Serve: Divide the avocado pasta among plates and top with tomatoes, spring onions, toasted pine nuts, and vegan feta as desired. Serve immediately.